Patent Publication Number: US-2012025208-A1

Title: Method for manufacturing silicon carbide substrate and silicon carbide substrate

Description:
TECHNICAL FIELD 
     The present invention relates to a method for manufacturing a silicon carbide substrate, and the silicon carbide substrate, more particularly, a method for manufacturing a silicon carbide substrate, and the silicon carbide substrate, each of which achieves reduced cost of manufacturing a semiconductor device using the silicon carbide substrate. 
     BACKGROUND ART 
     In recent years, in order to achieve high breakdown voltage, low loss, and utilization of semiconductor devices under a high temperature environment, silicon carbide (SiC) has begun to be adopted as a material for a semiconductor device. Silicon carbide is a wide band gap semiconductor having a band gap larger than that of silicon, which has been conventionally widely used as a material for semiconductor devices. Hence, by adopting silicon carbide as a material for a semiconductor device, the semiconductor device can have a high breakdown voltage, reduced on-resistance, and the like. Further, the semiconductor device thus adopting silicon carbide as its material has characteristics less deteriorated even under a high temperature environment than those of a semiconductor device adopting silicon as its material, advantageously. 
     Under such circumstances, various studies have been conducted on methods for manufacturing silicon carbide crystals and silicon carbide substrates used for manufacturing of semiconductor devices, and various ideas have been proposed (for example, see M. Nakabayashi, et al., “Growth of Crack-free 100 mm-diameter 4H-SiC Crystals with Low Micropipe Densities, Mater. Sci. Forum, vols. 600-603, 2009, p. 3-6 (Non-Patent Literature 1)). 
     CITATION LIST 
     Non Patent Literature 
     
         
         NPL 1: M. Nakabayashi, et al., “Growth of Crack-free 100 mm-diameter 4H-SiC Crystals with Low Micropipe Densities, Mater. Sci. Forum, vols. 600-603, 2009, p. 3-6 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, silicon carbide does not have a liquid phase at an atmospheric pressure. In addition, crystal growth temperature thereof is 2000° C. or greater, which is very high. This makes it difficult to control and stabilize growth conditions. Accordingly, it is difficult for a silicon carbide single-crystal to have a large diameter while maintaining its quality to be high. Hence, it is not easy to obtain a high-quality silicon carbide substrate having a large diameter. This difficulty in fabricating such a silicon carbide substrate having a large diameter results in not only increased manufacturing cost of the silicon carbide substrate but also fewer semiconductor devices produced for one batch using the silicon carbide substrate. Accordingly, manufacturing cost of the semiconductor devices is increased, disadvantageously. It is considered that the manufacturing cost of the semiconductor devices can be reduced by effectively utilizing a silicon carbide single-crystal, which is high in manufacturing cost, as a substrate. 
     In view of this, an object of the present invention is to provide a method for manufacturing a silicon carbide substrate, and the silicon carbide substrate, each of which achieves reduced cost of manufacturing a semiconductor device using the silicon carbide substrate. 
     Solution to Problem 
     A method for manufacturing a silicon carbide substrate in the present invention includes the steps of: preparing a base substrate made of silicon carbide and a SiC substrate made of single-crystal silicon carbide; forming a Si film made of silicon on and in contact with a main surface of the base substrate; fabricating a stacked substrate by placing the SiC substrate on and in contact with the Si film; and connecting the base substrate and the SiC substrate to each other by heating the stacked substrate to convert, into silicon carbide, at least a region making contact with the base substrate and a region making contact with the SiC substrate in the Si film. 
     As described above, it is difficult for a high-quality silicon carbide single-crystal to have a large diameter. Meanwhile, for efficient manufacturing in a process of manufacturing a semiconductor device using a silicon carbide substrate, a substrate provided with predetermined uniform shape and size is required. Hence, even when a high-quality silicon carbide single-crystal (for example, silicon carbide single-crystal having a small defect density) is obtained, a region that cannot be processed into such a predetermined shape and the like by cutting, etc., may not be effectively used. 
     To address this, in the method for manufacturing the silicon carbide substrate of the present invention, the SiC substrate made of single-crystal silicon carbide different from that of the base substrate is connected onto the base substrate. Thus, the silicon carbide substrate can be manufactured, for example, in the following manner. That is, the base substrate formed of low-quality silicon carbide crystal having a large defect density is processed to have the predetermined shape and size. On such a base substrate, a high-quality silicon carbide single-crystal not shaped into the predetermined shape and the like is employed as the SiC substrate. Then, they are connected to each other. The silicon carbide substrate manufactured through such a process has the predetermined uniform shape and size, thereby achieving efficient manufacturing of semiconductor devices. Further, the silicon carbide substrate manufactured through such a process utilizes the SiC substrate formed of high-quality silicon carbide single-crystal and having not been used because it cannot be processed into a desired shape and the like conventionally. Using such a silicon carbide substrate, semiconductor devices can be manufactured, thereby effectively using silicon carbide single-crystal. Furthermore, in the method for manufacturing the silicon carbide substrate in the present invention, at least the portions of the Si film are converted into silicon carbide, thereby obtaining an intermediate layer allowing the base substrate and the SiC substrate to be firmly connected to each other. Hence, the silicon carbide substrate can be handled as one freestanding substrate. As such, according to the method for manufacturing the silicon carbide substrate in the present invention, there can be manufactured a silicon carbide substrate that allows for reduced cost of manufacturing semiconductor devices using the silicon carbide substrate. 
     Preferably, the method for manufacturing the silicon carbide substrate further includes the step of smoothing at least one of main surfaces of the base substrate and the SiC substrate, which are to be disposed face to face with each other with the Si film interposed therebetween in the step of fabricating the stacked substrate, the step of smoothing being performed before the step of fabricating the stacked substrate. 
     Thus, the surface to serve as the connection surface is smoothed in advance, thereby allowing the base substrate and the SiC substrate to be connected to each other more securely. In order to attain further secure connection between the base substrate and the SiC substrate, it is preferable to smooth both the main surfaces of the base substrate and the SiC substrate, which are to be disposed face to face with the Si film interposed therebetween in the step of fabricating the stacked substrate. 
     Preferably, in the method for manufacturing the silicon carbide substrate, the Si film formed in the step of forming the Si film has a thickness of not less than 10 nm and not more than 1 μm. 
     If the thickness of the Si film formed on the base substrate is less than 10 nm and surface smoothness of each of the surfaces of the base substrate and the SiC substrate is not sufficiently high, the Si film to be formed between the base substrate and the SiC substrate becomes discontinuous, which may result in failure in achieving firm connection between the base substrate and the SiC substrate. In contrast, if the thickness of the Si film is more than 1 μm, the thickness of the intermediate layer (layer obtained by converting at least the portions of the Si film into silicon carbide) in the thickness of the silicon carbide substrate to be manufactured becomes large. This may result in decreased characteristics particularly when fabricating a vertical type device in which a current flows in the thickness direction of silicon carbide substrate  1 . Hence, the Si film formed preferably has a thickness of not less than 10 nm and not more than 1 μm. 
     Preferably, in the method for manufacturing the silicon carbide substrate, in the step of connecting the base substrate and the SiC substrate to each other, the stacked substrate is heated in an atmosphere including a gas containing carbon. 
     Accordingly, carbon is supplied to the Si film not only from the base substrate and the SiC substrate but also from the atmosphere, thereby achieving efficient conversion of silicon of the Si film into silicon carbide. 
     Preferably, in the method for manufacturing the silicon carbide substrate, in the step of fabricating the stacked substrate, a plurality of the SiC substrates are arranged side by side when viewed in a planar view. 
     As described above, it is difficult for a high-quality silicon carbide single-crystal to have a large diameter. To address this, the plurality of SiC substrates each obtained from a high-quality silicon carbide single-crystal are arranged side by side on the base substrate having a large diameter when viewed in a planar view, thereby obtaining a silicon carbide substrate that can be handled as a substrate having a high-quality SiC layer and a large diameter. By using such a silicon carbide substrate, the process of manufacturing a semiconductor device can be improved in efficiency. It should be noted that in order to further improve the efficiency of the process of manufacturing a semiconductor device, it is preferable that adjacent ones of the plurality of SiC substrates are arranged in contact with one another. More specifically, for example, the plurality of SiC substrates are preferably arranged in contact with one another in the form of a matrix. 
     In the method for manufacturing the silicon carbide substrate, in the stacked substrate, a main surface of the SiC substrate opposite to the base substrate has an off angle of not less than 50° and not more than 65° relative to a {0001} plane. 
     By growing single-crystal silicon carbide of hexagonal system in the &lt;0001&gt; direction, a high-quality single-crystal can be fabricated efficiently. From such a silicon carbide single-crystal grown in the &lt;0001&gt; direction, a silicon carbide substrate having a main surface corresponding to the {0001} plane can be obtained efficiently. Meanwhile, by using a silicon carbide substrate having a main surface having an off angle of not less than 50° and not more than 65° relative to the plane orientation of {0001}, a semiconductor device with high performance may be manufactured. 
     Specifically, for example, it is general that a silicon carbide substrate used for fabrication of a MOSFET has a main surface having an off angle of approximately 8° relative to the plane orientation of {0001}. An epitaxial growth layer is formed on this main surface and an oxide film, an electrode, and the like are formed on this epitaxial growth layer, thereby obtaining a MOSFET. In this MOSFET, a channel region is formed in a region including an interface between the epitaxial growth layer and the oxide film. However, in the MOSFET having such a structure, a multiplicity of interface states are formed around the interface between the epitaxial growth layer and the oxide film, i.e., the location in which the channel region is formed, due to the substrate&#39;s main surface having an off angle of approximately 8° relative to the {0001} plane. This hinders traveling of carriers, thus decreasing channel mobility. 
     To address this, in the stacked substrate, by setting the main surface of the SiC substrate opposite to the base substrate to have an off angle of not less than 50° and not more than 65° relative to the {0001} plane, the silicon carbide substrate to be manufactured will have a main surface having an off angle of not less than 50° and not more than 65° relative to the {0001} plane. This reduces formation of interface states. Hence, a MOSFET with reduced on-resistance can be fabricated. 
     In the method for manufacturing the silicon carbide substrate, in the stacked substrate, the main surface of the SiC substrate opposite to the base substrate has an off orientation forming an angle of not more than 5° relative to the &lt;1-100&gt; direction. 
     The &lt;1-100&gt; direction is a representative off orientation in a silicon carbide substrate. Variation in the off orientation resulting from variation in a slicing process of the process of manufacturing the substrate is adapted to be not more than 5°, which allows an epitaxial growth layer to be formed readily on the silicon carbide substrate. 
     In the above-described method for manufacturing the silicon carbide substrate, in the stacked substrate, the main surface of the SiC substrate opposite to the base substrate can have an off angle of not less than −3° and not more than 5° relative to a {03-38} plane in the &lt;1-100&gt; direction. 
     Accordingly, channel mobility can be further improved in the case where a MOSFET is fabricated using the silicon carbide substrate. Here, setting the off angle at not less than −3° and not more than +5° relative to the plane orientation of {03-38} is based on a fact that particularly high channel mobility was obtained in this set range as a result of inspecting a relation between the channel mobility and the off angle. 
     Further, the “off angle relative to the {03-38} plane in the &lt;1-100&gt; direction” refers to an angle formed by an orthogonal projection of a normal line of the above-described main surface to a flat plane defined by the &lt;1-100&gt; direction and the &lt;0001&gt; direction, and a normal line of the {03-38} plane. The sign of positive value corresponds to a case where the orthogonal projection approaches in parallel with the &lt;1-100&gt; direction whereas the sign of negative value corresponds to a case where the orthogonal projection approaches in parallel with the &lt;0001&gt; direction. 
     It should be noted that the main surface preferably has a plane orientation of substantially {03-38}, and the main surface more preferably has a plane orientation of {03-38}. Here, the expression “the main surface has a plane orientation of substantially {03-38}” is intended to encompass a case where the plane orientation of the main surface of the substrate is included in a range of off angle such that the plane orientation can be substantially regarded as {03-38} in consideration of processing accuracy of the substrate. In this case, the range of off angle is, for example, a range of off angle of ±2° relative to {03-38}. Accordingly, the above-described channel mobility can be further improved. 
     In the method for manufacturing the silicon carbide substrate, in the stacked substrate, the main surface of the SiC substrate opposite to the base substrate has an off orientation forming an angle of not more than 5° relative to the &lt;11-20&gt; direction. 
     The &lt;11-20&gt; direction is a representative off orientation in a silicon carbide substrate, as with the &lt;1-100&gt; direction. Variation in the off orientation resulting from variation in the slicing process of the process of manufacturing the substrate is adapted to be ±5°, which allows an epitaxial growth layer to be formed readily on the SiC substrate. 
     In the method for manufacturing the silicon carbide substrate, the base substrate may be made of single-crystal silicon carbide, and in the step of fabricating the stacked substrate, the stacked substrate may be fabricated such that main surfaces of the base substrate and the SiC substrate, which are disposed face to face with each other with the Si film interposed therebetween, have the same plane orientation. 
     A thermal expansion coefficient of single-crystal silicon carbide is anisotropic depending on its crystal plane. Hence, when surfaces corresponding to crystal planes greatly different from each other in thermal expansion coefficient are connected to each other, stress resulting from the difference in thermal expansion coefficient is applied between the base substrate and the SiC substrate. This stress may cause strains or cracks of the silicon carbide substrate in the manufacturing of the silicon carbide substrate or in the process of manufacturing semiconductor devices using the silicon carbide substrate. To address this, the silicon carbide single-crystals to constitute the above-described connection surface are adapted to have the same plane orientation, thereby reducing the stress. It should be noted that the state in which “the main surfaces of the base substrate and the SiC substrate have the same plane orientation” does not need to correspond to a state in which the plane orientations of the main surfaces are strictly the same, and may correspond to a state in which they are substantially the same. More specifically, when the crystal plane constituting the main surface of the base substrate forms an angle of not more than 1′ relative to the crystal plane constituting the main surface of the SiC substrate, it can be said that the main surfaces of the base substrate and the SiC substrate has substantially the same plane orientation. 
     In the method for manufacturing the silicon carbide substrate, in the stacked substrate, the main surface of the SiC substrate opposite to the base substrate has an off angle of not less than 1° and not more than 60° relative to the {0001} plane. 
     By growing a silicon carbide single-crystal of hexagonal system in the &lt;0001&gt; direction as described above, a high-quality single-crystal can be fabricated efficiently. From such a silicon carbide single-crystal grown in the &lt;0001&gt; direction, SiC substrates can be obtained relatively effectively so far as the surface does not have a large off angle relative to the {0001} plane, specifically, has an off angle of 60° or smaller. Meanwhile, with the off angle being 1° or greater, a high-quality epitaxial growth layer can be formed on such a SiC substrate. 
     In the method for manufacturing the silicon carbide substrate, the step of connecting the base substrate and the SiC substrate to each other is performed without polishing main surfaces of the base substrate and the SiC substrate before the step of connecting the base substrate and the SiC substrate to each other, the main surfaces of the base substrate and the SiC substrate being to be disposed face to face with each other in the step of connecting the base substrate and the SiC substrate to each other. 
     Accordingly, the manufacturing cost of the silicon carbide substrate can be reduced. Here, as described above, the main surfaces of the base substrate and the SiC substrate, which are to be disposed face to face with each other in the step of connecting the base substrate and the SiC substrate to each other, may not be polished. However, for removal of damaged layers in the vicinity of surfaces formed by slicing upon fabricating the substrate, it is preferable to perform the step of connecting the base substrate and the SiC substrate to each other, after performing a step of removing the damaged layers by means of etching, for example. 
     The method for manufacturing the silicon carbide substrate may further include the step of polishing a main surface of the SiC substrate, the main surface corresponding to a main surface of the SiC substrate to be opposite to the base substrate. 
     This allows a high-quality epitaxial growth layer to be formed on the main surface of the SiC substrate opposite to the base substrate. As a result, a semiconductor device can be manufactured which includes the high-quality epitaxial growth layer as an active layer, for example. Namely, by employing such a step, a silicon carbide substrate can be obtained which allows for manufacturing of a high-quality semiconductor device including the epitaxial growth layer formed on the SiC substrate. Here, the main surface of the SiC substrate may be polished after connecting the base substrate and the SiC substrate to each other, or before connecting the base substrate and the SiC substrate to each other by previously polishing the main surface of the SiC substrate, which is to be opposite to the base substrate. 
     A silicon carbide substrate according to the present invention includes: a base layer made of silicon carbide; an intermediate layer formed on and in contact with the base layer; and a SiC layer made of single-crystal silicon carbide and disposed on and in contact with the intermediate layer. The intermediate layer contains silicon carbide at least at its region adjacent to the base layer and its region adjacent to the SiC layer and connects the base layer and the SiC layer to each other. The silicon carbide in the region adjacent to the base layer and the region adjacent to the SiC layer may be amorphous. 
     In the silicon carbide substrate of the present invention, the SiC layer made of single-crystal silicon carbide different from that of the base layer is connected onto the base layer. Hence, for example, a low-quality silicon carbide crystal having a large defect density is processed into predetermined shape and size suitable for manufacturing of semiconductor devices to serve as the base layer, whereas a high-quality silicon carbide single-crystal having a suitable shape and the like for manufacturing of semiconductor devices is disposed on the base layer as the SiC layer. Such a silicon carbide substrate have the predetermined uniform shape and size, thus attaining effective manufacturing of semiconductor devices. Further, semiconductor devices can be manufactured using such a silicon carbide substrate that employs the high-quality SiC layer thus having a difficulty in being processed into the shape and the like suitable for manufacturing of semiconductor devices, thereby effectively utilizing the silicon carbide single-crystal. Further, in the silicon carbide substrate of the present invention, the base layer and the SiC layer are connected to each other and are unified by the intermediate layer containing silicon carbide at its region adjacent to the base layer and its region adjacent to the SiC layer. Hence, the silicon carbide substrate can be handled as one freestanding substrate. As such, according to the silicon carbide substrate of the present invention, there can be provided a silicon carbide substrate allowing for reduced cost of manufacturing semiconductor devices using the silicon carbide substrate. 
     In the silicon carbide substrate, preferably, a plurality of the SiC layers are arranged side by side when viewed in a planar view. 
     Thus, the plurality of SiC layers each obtained from a high-quality silicon carbide single-crystal are arranged side by side on the base layer having a large diameter when viewed in a planar view, thereby obtaining a silicon carbide substrate that can be handled as a substrate having a high-quality SiC layer and a large diameter. By using such a silicon carbide substrate, the process of manufacturing a semiconductor device can be improved in efficiency. It should be noted that in order to improve the efficiency of the process of manufacturing a semiconductor device, it is preferable that adjacent ones of the plurality of SiC layers are arranged in contact with one another. More specifically, for example, the plurality of SiC layers are preferably arranged in contact with one another in the form of a matrix. 
     In the silicon carbide substrate, the base layer may be made of single-crystal silicon carbide. In this case, no micro pipe of the base layer is preferably propagated to the SiC layer. 
     As the base layer, single-crystal silicon carbide having relatively many defects such as micro pipes can be employed. In employing it, the micro pipes formed in the base layer are prevented from being propagated to the SiC layer, thereby allowing a high-quality epitaxial growth layer to be formed on the SiC layer. The silicon carbide substrate of the present invention can be fabricated by connecting a separately grown SiC layer onto the base layer instead of directly growing the SiC layer on the base layer. Thus, the micro pipes formed in the base layer can be readily prevented from being propagated to the SiC layer. 
     In the silicon carbide substrate, a main surface of the SiC layer opposite to the base layer has an off angle of not less than 50° and not more than 65° relative to a {0001} plane. 
     As such, in the silicon carbide substrate of the present invention, the main surface of the SiC layer opposite to the base layer is adapted to have an off angle of not less than 50° and not more than 65° relative to the {0001} plane, thereby reducing formation of interface states around an interface between an epitaxial growth layer and an oxide film, i.e., a location where a channel region is formed upon forming a MOSFET using the silicon carbide substrate, for example. Accordingly, a MOSFET with reduced on-resistance can be fabricated. 
     In the silicon carbide substrate, the main surface of the SiC layer opposite to the base layer may have an off orientation forming an angle of not more than 5° relative to the &lt;1-100&gt; direction. 
     The &lt;1-100&gt; direction is a representative off orientation in a silicon carbide substrate. Variation in the off orientation resulting from variation in a slicing process of the process of manufacturing the substrate is adapted to be 5° or smaller, which allows an epitaxial growth layer to be formed readily on the silicon carbide substrate. 
     In the silicon carbide substrate, the main surface of the SiC layer opposite to the base layer has an off angle of not less than −3° and not more than 5° relative to the {03-38} plane in the &lt;1-100&gt; direction. 
     Accordingly, channel mobility can be further improved in the case where a MOSFET is fabricated using the silicon carbide substrate. Here, the “off angle relative to the {03-38} plane in the &lt;1-100&gt; direction” refers to an angle formed by an orthogonal projection of a normal line of the above-described main surface to a flat plane defined by the &lt;1-100&gt; direction and the &lt;0001&gt; direction, and a normal line of the {03-38} plane. The sign of positive value corresponds to a case where the orthogonal projection approaches in parallel with the &lt;1-100&gt; direction whereas the sign of negative value corresponds to a case where the orthogonal projection approaches in parallel with the &lt;0001&gt; direction. 
     Further, the main surface preferably has a plane orientation of substantially {03-38}, and the main surface more preferably has a plane orientation of {03-38}. Here, the expression “the main surface has a plane orientation of substantially {03-38}” is intended to encompass a case where the plane orientation of the main surface of the substrate is included in a range of off angle such that the plane orientation can be substantially regarded as {03-38} in consideration of processing accuracy of the substrate. In this case, the range of off angle is, for example, a range of off angle of +2° relative to {03-38}. Accordingly, the above-described channel mobility can be further improved. 
     In the silicon carbide substrate, the main surface of the SiC layer opposite to the base layer has an off orientation forming an angle of not more than 5° relative to the &lt;11-20&gt; direction. 
     The &lt;11-20&gt; direction is a representative off orientation in a silicon carbide substrate, as with the &lt;1-100&gt; direction. Variation in the off orientation resulting from variation in a slicing process of the process of manufacturing the substrate is adapted to be +5°, which allows an epitaxial growth layer to be formed readily on silicon carbide substrate  1 . 
     In the silicon carbide substrate, the base layer may be made of single-crystal silicon carbide. In this case, the main surfaces of the base layer and the SiC layer, which are disposed face to face with each other with the intermediate layer interposed therebetween, preferably has the same plane orientation. 
     This suppresses stress resulting from anisotropy in thermal expansion coefficient depending on a crystal plane to exert between the base layer and the SiC layer. It should be noted that the state in which the main surfaces of the base layer and the SiC layer have the same plane orientation” does not need to correspond to a state in which the plane orientations of the main surfaces are strictly the same, and may correspond to a state in which they are substantially the same. More specifically, it can be said that the main surfaces of the base layer and the SiC layer has substantially the same plane orientation as long as the crystal plane constituting the main surface of the base layer forms an angle of 1° or smaller relative to the crystal plane constituting the SiC layer. 
     In the silicon carbide substrate, the main surface of the SiC layer opposite to the base layer may have an off angle of not less than 1° and not more than 60° relative to a {0001} plane. 
     As described above, from the silicon carbide single-crystal grown in the &lt;0001&gt; direction, single-crystal silicon carbide having a large off angle relative to the {0001} plane, specifically, having an off angle of 60° or smaller can be obtained relatively efficiently and can be employed as the SiC layer. Meanwhile, with the off angle being 1° or greater, a high-quality epitaxial growth layer can be readily formed on such a SiC substrate. 
     In the silicon carbide substrate, the main surface of the SiC layer opposite to the base layer may be polished. This allows a high-quality epitaxial growth layer to be formed on the main surface of the SiC layer opposite to the base layer. As a result, a semiconductor device can be manufactured which includes the high-quality epitaxial growth layer as an active layer, for example. Namely, by employing such a structure, the silicon carbide substrate can be obtained which allows for manufacturing of a high-quality semiconductor device including the epitaxial layer formed on the SiC layer. 
     Advantageous Effects of Invention 
     As apparent from the description above, a method for manufacturing a silicon carbide substrate, and the silicon carbide substrate in the present invention provides a method for manufacturing a silicon carbide substrate, and the silicon carbide substrate, each of which achieves reduced cost of manufacturing a semiconductor device using the silicon carbide substrate. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross sectional view showing a structure of a silicon carbide substrate. 
         FIG. 2  is a schematic cross sectional view showing the structure of the silicon carbide substrate having an epitaxial layer formed thereon. 
         FIG. 3  is a flowchart schematically showing a method for manufacturing the silicon carbide substrate. 
         FIG. 4  is a schematic cross sectional view for illustrating the method for manufacturing the silicon carbide substrate. 
         FIG. 5  is a schematic cross sectional view showing another structure of the silicon carbide substrate. 
         FIG. 6  is a schematic plan view showing the another structure of the silicon carbide substrate. 
         FIG. 7  is a schematic cross sectional view showing still another structure of the silicon carbide substrate. 
         FIG. 8  is a schematic cross sectional view showing a structure of a vertical type MOSFET. 
         FIG. 9  is a flowchart schematically showing a method for manufacturing the vertical type MOSFET. 
         FIG. 10  is a schematic cross sectional view for illustrating the method for manufacturing the vertical type MOSFET. 
         FIG. 11  is a schematic cross sectional view for illustrating the method for manufacturing the vertical type MOSFET. 
         FIG. 12  is a schematic cross sectional view for illustrating the method for manufacturing the vertical type MOSFET. 
         FIG. 13  is a schematic cross sectional view for illustrating the method for manufacturing the vertical type MOSFET. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes embodiments of the present invention with reference to figures. It should be noted that in the below-mentioned figures, the same or corresponding portions are given the same reference characters and are not described repeatedly. 
     First Embodiment 
     Referring to  FIG. 1 , silicon carbide substrate  1  in the present embodiment includes: a base layer  10  made of silicon carbide; an intermediate layer  40  formed on and in contact with base layer  10 ; and a SiC layer  20  made of single-crystal silicon carbide and disposed on and in contact with intermediate layer  40 . Intermediate layer  40  contains silicon carbide at least at its region adjacent to base layer  10  and its region adjacent to SiC layer  20 , and connects base layer  10  and SiC layer  20  to each other. The silicon carbide in each of the region adjacent to base layer  10  and the region adjacent to SiC layer  20  may be amorphous. 
     Then, when an epitaxial growth layer  60  made of single-crystal silicon carbide is formed on main surface  20 A of SiC layer  20  opposite to base layer  10  as shown in  FIG. 2 , stacking faults that can be generated in base layer  10  are not propagated to epitaxial growth layer  60 . Accordingly, stacking fault density in epitaxial growth layer  60  can be readily made smaller than that in base layer  10 . 
     In silicon carbide substrate  1  in the present embodiment, SiC layer  20 , which is made of single-crystal silicon carbide different from that of base layer  10 , is connected onto base layer  10 . Hence, for example, a low-quality silicon carbide crystal having a large defect density is processed to have a shape and a size suitable for the process of manufacturing a semiconductor device and is then employed as base layer  10 . On the other hand, a high-quality silicon carbide single-crystal not having a shape suitable for the process of manufacturing a semiconductor device can be disposed on base layer  10  as SiC layer  20 . This silicon carbide substrate  1  is uniformly shaped and sized appropriately, thereby achieving efficient manufacturing of semiconductor devices. Further, because the high-quality silicon carbide single-crystal having a difficulty in being processed into a shape suitable for the process of manufacturing can be used as SiC layer  20  in silicon carbide substrate  1  to manufacture a semiconductor device, thereby effectively utilizing the silicon carbide single-crystal. Further, in silicon carbide substrate  1 , base layer  10  and SiC layer  20  are unified by being connected to each other by intermediate layer  40  containing silicon carbide at its regions adjacent to base layer  10  and adjacent to SiC layer  20 . Hence, silicon carbide substrate  1  can be handled as one freestanding substrate. As such, silicon carbide substrate  1  described above allows for reduced cost in manufacturing semiconductor devices. Because intermediate layer  40  thus includes silicon carbide at least at its regions adjacent to base layer  10  and adjacent to SiC layer  20 , base layer  10  and SiC layer  20  are connected to each other more firmly. 
     Here, base layer  10  can adopt a structure from various structures as long as it is made of silicon carbide. For example, base layer  10  may be of, for example, polycrystal silicon carbide or a sintered compact of silicon carbide. Alternatively, base layer  10  may be made of single-crystal silicon carbide. In this case, it is preferable that no micro pipes in base layer  10  are propagated to SiC layer  20 . Further, in the case where silicon carbide substrate  1  is employed to manufacture a semiconductor device in which a current flows in the thickness direction of silicon carbide substrate  1 , base layer  10  preferably has a small resistivity. Specifically, base layer  10  preferably has a resistivity of 50 mΩcm or smaller, more preferably, 10 mΩcm or smaller. 
     In the case where single-crystal silicon carbide containing relatively many defects such as micro pipes is employed as base layer  10 , a high-quality epitaxial growth layer can be formed on SiC layer  20  by preventing the micro pipes formed in base layer  10  from being propagated to SiC layer  20 . Silicon carbide substrate  1  in the present embodiment can be fabricated by connecting SiC layer  20 , which has not been grown on base layer  10  and has grown separately therefrom, onto base layer  10 . Hence, it is easy to prevent the micro pipes formed in base layer  10  from being propagated to SiC layer  20 . 
     Further, in the case where base layer  10  is made of single-crystal silicon carbide, it is preferable that the main surface of base layer  10 , which faces SiC layer  20  with intermediate layer  40  interposed therebetween, has the same plane orientation as that of the main surface of SIC layer  20 . This suppresses stress resulting from anisotropy in thermal expansion coefficient to exert between base layer  10  and SiC layer  20 . 
     Further, in silicon carbide substrate  1  described above, main surface  20 A of SiC substrate  20  opposite to base layer  10  may have an off angle of not less than 50° and not more than 65° relative to the {0001} plane. Accordingly, when fabricating a MOSFET using silicon carbide substrate  1 , formation of interface states is reduced around an interface between an epitaxial growth layer and an oxide film thereof, i.e., a location where a channel region is formed. In this way, the MOSFET fabricated has reduced on-resistance. 
     Further, in silicon carbide substrate  1 , the off orientation of main surface  20 A may form an angle of 5° or smaller relative to the &lt;1-100&gt; direction. The &lt;1-100&gt; direction is a representative off orientation in a silicon carbide substrate. Variation in the off orientation resulting from variation in a slicing process of the process of manufacturing the substrate is adapted to be 5° or smaller, which allows an epitaxial growth layer to be formed readily on silicon carbide substrate  1 . 
     Further, in the silicon carbide substrate, main surface  20 A may have an off angle of not less than −3° and not more than 5° relative to the {03-38} plane in the &lt;1-100&gt; direction. Accordingly, channel mobility can be further improved in the case where a MOSFET is fabricated using silicon carbide substrate  1 . 
     Meanwhile, in silicon carbide substrate  1 , the off orientation of main surface  20 A may form an angle of 5° or smaller relative to the &lt;11-20&gt; direction. The &lt;11-20&gt; direction is a representative off orientation in a silicon carbide substrate, as with the &lt;1-100&gt; direction. Variation in the off orientation resulting from variation in a slicing process of the process of manufacturing the substrate is adapted to be ±5°, which allows an epitaxial growth layer to be formed readily on silicon carbide substrate  1 . 
     Further, in silicon carbide substrate  1 , main surface  20 A may have an off angle of not less than 1° and not more than 60° relative to the {0001} plane. This allows a silicon carbide single-crystal usable as SiC layer  20  to be obtained effectively, and facilitates formation of a high-quality epitaxial growth layer on SiC layer  20 . 
     Further, for ease of handling as a freestanding substrate, silicon carbide substrate  1  preferably has a thickness of 300 μm or greater. Further, when silicon carbide substrate  1  is employed to fabricate a power device, SiC layer  20  preferably has a polytype of 4H. 
     Further, in silicon carbide substrate  1 , main surface  20 A of SiC layer  20  opposite to base layer  10  is preferably polished. This allows for formation of a high-quality epitaxial growth layer on main surface  20 A. As a result, a semiconductor device can be manufactured which includes the high-quality epitaxial growth layer as an active layer, for example. Namely, by employing such a structure, silicon carbide substrate  1  can be obtained which allows for manufacturing of a high-quality semiconductor device including the epitaxial layer formed on SiC layer  20 . 
     The following describes an exemplary method for manufacturing silicon carbide substrate  1  described above. Referring to  FIG. 3 , in the method for manufacturing the silicon carbide substrate in the present embodiment, first, as a step (S 10 ), a substrate preparing step is perfolined. In this step (S 10 ), referring to  FIG. 4 , a base substrate  10  formed of silicon carbide and a SiC substrate  20  formed of single-crystal silicon carbide are prepared. SiC substrate  20  has the main surface, which will be main surface  20 A of SiC layer  20  that will be obtained by this manufacturing method (see  FIG. 1 ). Hence, on this occasion, the plane orientation of the main surface of SiC substrate  20  is selected in accordance with desired plane orientation of main surface  20 A. Here, for example, a SiC substrate  20  having a main surface corresponding to the {03-38} plane is prepared. 
     Meanwhile, for base substrate  10 , a substrate having an impurity density greater than that of SiC substrate  20  is employed, such as a substrate having an impurity density greater than 2×10 19  cm −3 . Here, the term “impurity” refers to an impurity introduced to generate majority carriers in the semiconductor substrates, i.e., base substrate  10  and SiC substrate  20 . A usable example thereof is nitrogen. Further, base substrate  10  preferably has a diameter of 2 inches or greater, more preferably, of 6 inches or greater in order to achieve efficient fabrication of semiconductor devices using silicon carbide substrate  1 . Further, in order to prevent generation of cracks between base substrate  10  and SiC substrate  20  in the process of manufacturing semiconductor devices using silicon carbide substrate  1 , it is preferable to reduce a difference in thermal expansion coefficient therebetween. Further, in order to reduce a difference between base substrate  10  and SiC substrate  20  in physical properties such as thermal expansion coefficient, base substrate  10  and SiC substrate  20  preferably have the same crystal structure (the same polytype). 
     Next, a substrate smoothing step is performed as a step (S 20 ). In this step (S 20 ), the respective main surfaces (connection surface) of base substrate  10  and SiC substrate  20 , which are to be disposed face to face with each other with a Si film interposed therebetween in a subsequent step (S 40 ), are smoothed by polishing, for example. It should be noted that although this step (S 20 ) is not an essential step, by performing this step, the Si film will be formed uniformly in a below-described step (S 30 ) to allow base substrate  10  and SiC substrate  20  to be connected to each other more securely in a step (S 50 ). Further, variation of the thickness of each of base substrate  10  and SiC substrate  20  (difference between the maximum value and the minimum value of the thickness) is preferably reduced as much as possible, specifically, is preferably 10 μm or smaller. 
     Meanwhile, step (S 20 ) may be omitted, i.e., step (S 30 ) may be performed without polishing the main surfaces of base substrate  10  and SiC substrate  20 , which are to face each other. This reduces manufacturing cost of silicon carbide substrate  1 . Further, for removal of damaged layers located in surfaces formed by slicing upon fabrication of base substrate  10  and SiC substrate  20 , a step of removing the damaged layers may be performed by, for example, etching instead of step (S 20 ) or after step (S 20 ), and then step (S 30 ) described below may be performed. 
     Next, a Si film forming step is performed as step (S 30 ). In this step (S 30 ), referring to  FIG. 4 , Si film  30  made of silicon is formed on the main surface of base substrate  10 . Si film  30  can be formed using a method such as a sputtering method, a deposition method, a liquid phase epitaxy, or a vapor phase epitaxy. Further, in forming Si film  30 , nitrogen, phosphorus, aluminum, boron, or the like can be doped as an impurity. Further, Si film  30  may be adapted to contain titanium to improve solid solubility of carbon in Si film  30  to facilitate conversion thereof into silicon carbide in the below-described step (S 50 ). 
     Next, a stacking step is performed as step (S 40 ). In this step (S 40 ), referring to 
       FIG. 4 , SiC substrate  20  is placed on and in contact with Si film  30  formed on and in contact with the main surface of base substrate  10 , thereby fabricating a stacked substrate. 
     Next, as step (S 50 ), a connecting step is performed. In step (S 50 ), base substrate  10  and SiC substrate  20  are connected to each other by heating the stacked substrate. More specifically, for example, the stacked substrate is heated for not less than 1 hour and not more than 30 hours to fall within a range of temperature from 1300° C. to 1800° C. In this way, carbon is supplied from base substrate  10  and SiC substrate  20  to Si film  30 , thereby converting at least portions of Si film  30  into silicon carbide. By performing the heating under a gas containing carbon atoms, for example, under an atmosphere including a hydrocarbon gas such as propane, ethane, or ethylene, carbon is supplied from the atmosphere to Si film  30  to facilitate the conversion of silicon constituting Si film  30  into silicon carbide. By heating the stacked substrate in this way, at least the region in contact with base substrate  10  and the region in contact with SiC substrate  20  in Si film  30  are converted into silicon carbide, thereby connecting base substrate  10  and SiC substrate  20  to each other. As a result, silicon carbide substrate  1  shown in  FIG. 1  is obtained. Further, the atmosphere upon the heating in step (S 50 ) may be inert gas atmosphere. In the case where the atmosphere is the inert gas atmosphere, the inert gas atmosphere preferably contains at least one selected from a group consisting of argon, helium, and nitrogen. Further, in this step (S 50 ), the stacked substrate may be heated in an atmosphere obtained by reducing pressure of the atmospheric air. This reduces manufacturing cost of silicon carbide substrate  1 . 
     Thus, in the method for manufacturing silicon carbide substrate  1  in the present embodiment, SiC substrate  20  made of single-crystal silicon carbide different from that of base substrate  10  is connected onto base substrate  10 . As such, base substrate  10  formed of an inexpensive, low-quality silicon carbide crystal having a large defect density can be processed to have a shape and a size suitable for manufacturing of semiconductor devices, whereas a high-quality silicon carbide single-crystal not having a shape and the like suitable for manufacturing of semiconductor devices can be disposed as SiC substrate  20  on base substrate  10 . Silicon carbide substrate  1  manufactured through such a process has the predetermined uniform shape and size. This allows for efficient manufacturing of semiconductor devices. Further, silicon carbide substrate  1  manufactured through such a process utilizes such a high-quality SiC substrate  20  (SiC layer  20 ) to manufacture a semiconductor device, thereby effectively utilizing silicon carbide single-crystal. Further, in the method for manufacturing silicon carbide substrate  1  in the present invention, base substrate  10  and SiC substrate  20  are firmly connected to each other by intermediate layer  40  formed by converting at least the portions of Si film  30  into silicon carbide. Hence, silicon carbide substrate  1  can be handled as one freestanding substrate. As such, according to the method for manufacturing silicon carbide substrate  1  in the present embodiment, there can be manufactured a silicon carbide substrate  1  that allows for reduced cost of manufacturing semiconductor devices using silicon carbide substrate  1 . 
     Further, by epitaxially growing single-crystal silicon carbide on silicon carbide substrate  1  to form an epitaxial growth layer  60  on main surface  20 A of SiC substrate  20 , a silicon carbide substrate  2  shown in  FIG. 2  can be manufactured. 
     Here, in step (S 30 ), the Si film formed preferably has a thickness of not less than 10 nm and not more than 1 μm. If the thickness of Si film  30  formed on base substrate  10  is less than 10 nm and surface smoothness of each of the surfaces of base substrate  10  and SiC substrate  20  is not sufficiently high, Si film  30  to be formed between base substrate  10  and SiC substrate  20  becomes discontinuous, which may lead to failure in achieving firm connection between base substrate  10  and SiC substrate  20 . In contrast, if the thickness of Si film  30  is more than 1 μm, the thickness of intermediate layer  40  in the thickness of silicon carbide substrate  1  becomes large. This may result in decreased characteristics particularly when fabricating a vertical type device in which a current flows in the thickness direction of silicon carbide substrate  1 . Thus, Si film  30  formed preferably has a thickness of not less than 10 nm and not more than 1 μm. 
     Further, in step (S 40 ), the stacked substrate is preferably fabricated such that the plane orientations of the main surfaces of base substrate  10  and SiC substrate  20 , which face each other with Si film  30  interposed therebetween, coincide with each other. This suppresses stress resulting from anisotropy in thermal expansion coefficient to exert between base substrate  10  and SiC substrate  20 . 
     Further, in step (S 50 ), Si film  30  (intermediate layer  40 ) may be doped with a desired impurity by adding nitrogen, trimethylaluminum, diborane, phosphine, or the like in the atmosphere in which the stacked substrate is heated. 
     In the above-described embodiment, it has been illustrated that: in the stacked substrate fabricated in step (S 40 ), main surface  20 A of SiC substrate  20  opposite to base substrate  10  has an off orientation corresponding to the &lt;1-100&gt; direction, and main surface  20 A thereof corresponds to the {03-38} plane. However, instead of this, the main surface may have an off orientation forming an angle of 5° or smaller relative to the &lt;11-20&gt; direction. Further, main surface  20 A may have an off angle of not less than 1° and not more than 60° relative to the {0001} plane. 
     Further, the above-described method for manufacturing silicon carbide substrate  1  in the present embodiment may further include a step of polishing the main surface of SiC substrate  20  that corresponds to main surface  20 A of SiC substrate  20  opposite to base substrate  10  in the stacked substrate. Accordingly, a silicon carbide substrate  1  is manufactured in which main surface  20 A of SiC layer  20  opposite to base layer  10  has been polished. Here, the step of polishing may be performed before or after connecting base substrate  10  and SiC substrate  20  to each other, as long as the step of polishing is performed after step (S 10 ). 
     Second Embodiment 
     The following describes another embodiment of the present invention, i.e., a second embodiment. Referring to  FIG. 5 ,  FIG. 6 , and  FIG. 1 , a silicon carbide substrate  1  in the second embodiment has basically the same configuration and provides basically the same effects as those of silicon carbide substrate  1  in the first embodiment. However, silicon carbide substrate  1  in the second embodiment is different from that of the first embodiment in that a plurality of SiC layers  20  are arranged side by side when viewed in a planar view. 
     Namely, referring to  FIG. 5  and  FIG. 6 , in silicon carbide substrate  1  of the second embodiment, the plurality of SiC layers  20  are arranged side by side when viewed in a planar view. In other words, the plurality of SiC layers  20  are arranged along main surface  10 A of base layer  10 . More specifically, the plurality of SiC layers  20  are arranged in the form of a matrix on base layer  10  such that adjacent SiC layers  20  are in contact with each other. Accordingly, silicon carbide substrate  1  of the present embodiment can be handled as a substrate having high-quality SiC layers  20  and a large diameter. Utilization of such a silicon carbide substrate  1  allows for efficient manufacturing process of semiconductor devices. It should be noted that silicon carbide substrate  1  in the second embodiment can be manufactured in a similar way to that in the first embodiment by arranging the plurality of SiC substrates  20  side by side on Si film  30  in step (S 40 ) in the first embodiment. It should be noted that there may be formed a space between adjacent SiC layers (SiC substrates)  20 . The space is preferably 100 μm or smaller, more preferably, 10 μm or smaller. 
     Further, in the second embodiment, it has been illustrated that the plurality of SiC layers  20  each having a planar shape of square (quadrangle) are disposed on base layer  10 , but the shape of each of SiC layers  20  is not limited to this. Specifically, referring to  FIG. 7 , the planar shapes of SiC layers  20  can be any shapes such as a hexagon shape, a trapezoidal shape, a rectangular shape, and a circular shape, or may be a combination thereof. 
     Third Embodiment 
     As a third embodiment, the following describes one exemplary semiconductor device fabricated using the above-described silicon carbide substrate of the present invention. Referring to  FIG. 8 , a semiconductor device  101  according to the present invention is a DiMOSFET (Double Implanted MOSFET) of vertical type, and has a substrate  102 , a buffer layer  121 , a breakdown voltage holding layer  122 , p regions  123 , n +  regions  124 , p +  regions  125 , an oxide film  126 , source electrodes  111 , upper source electrodes  127 , a gate electrode  110 , and a drain electrode  112  formed on the backside surface of substrate  102 . Specifically, buffer layer  121  made of silicon carbide is formed on the front-side surface of substrate  102  made of silicon carbide of n type conductivity. Employed as substrate  102  is a silicon carbide substrate of the present invention, inclusive of silicon carbide substrate  1  described in each of the first and second embodiments. In the case where silicon carbide substrate  1  in each of the first and second embodiments is employed, buffer layer  121  is formed on SiC layer  20  of silicon carbide substrate  1 . Buffer layer  121  has n type conductivity, and has a thickness of, for example, 0.5 μm. Further, impurity with n type conductivity in buffer layer  121  has a density of, for example, 5×10 17  cm −3 . Formed on buffer layer  121  is breakdown voltage holding layer  122 . Breakdown voltage holding layer  122  is made of silicon carbide of n type conductivity, and has a thickness of 10 μm, for example. Further, breakdown voltage holding layer  122  includes an impurity of n type conductivity at a density of, for example, 5×10 15  cm −3 . 
     Breakdown voltage holding layer  122  has a surface in which p regions  123  of p type conductivity are formed with a space therebetween. In each of p regions  123 , an n +  region  124  is formed at the surface layer of p region  123 . Further, at a location adjacent to n +  region  124 , a p +  region  125  is formed. Oxide film  126  is formed to extend on n +  region  124  in one p region  123 , p region  123 , an exposed portion of breakdown voltage holding layer  122  between the two p regions  123 , the other p region  123 , and n +  region  124  in the other p region  1210   n  oxide film  126 , gate electrode  110  is formed. Further, source electrodes  111  are formed on n +  regions  124  and p +  regions  125 . On source electrodes  111 , upper source electrodes  127  are formed. Moreover, drain electrode  112  is formed on the backside surface of substrate  102 , i.e., the surface opposite to its front-side surface on which buffer layer  121  is formed. 
     Employed as substrate  102  in semiconductor device  101  of the present embodiment is a silicon carbide substrate of the present invention such as silicon carbide substrate  1  described above in the first and second embodiments. Here, as described above, the silicon carbide substrate of the present invention allows for reduced manufacturing cost of semiconductor devices. Hence, semiconductor device  101  is manufactured with the reduced manufacturing cost. 
     The following describes a method for manufacturing semiconductor device  101  shown in  FIG. 8 , with reference to  FIG. 9-FIG .  13 . Referring to  FIG. 9 , first, a substrate preparing step (S 110 ) is performed. Prepared here is, for example, substrate  102 , which is made of silicon carbide and has its main surface corresponding to the (03-38) plane (see  FIG. 10 ). As substrate  102 , there is prepared a silicon carbide substrate of the present invention, inclusive of silicon carbide substrate  1  manufactured in accordance with each of the manufacturing methods described in the first and second embodiments. 
     Alternatively, as substrate  102  (see  FIG. 10 ), a substrate may be employed which has n type conductivity and has a substrate resistance of 0.02 Ωcm. 
     Next, as shown in  FIG. 9 , an epitaxial layer forming step (S 120 ) is performed. Specifically, buffer layer  121  is formed on the front-side surface of substrate  102 . Buffer layer  121  is formed on SiC layer  20  (see  FIG. 1  and  FIG. 5 ) of silicon carbide substrate  1  employed as substrate  102 . As buffer layer  121 , an epitaxial layer is formed which is made of silicon carbide of n type conductivity and has a thickness of 0.5 μm, for example. Buffer layer  121  has a conductive impurity at a density of, for example, 5×10 17  cm −3 . Then, on buffer layer  121 , breakdown voltage holding layer  122  is formed as shown in  FIG. 10 . As breakdown voltage holding layer  122 , a layer made of silicon carbide of n type conductivity is formed using an epitaxial growth method. Breakdown voltage holding layer  122  can have a thickness of, for example, 10 μm. Further, breakdown voltage holding layer  122  includes an impurity of n type conductivity at a density of, for example, 5×10 15  cm −3 . 
     Next, as shown in  FIG. 9 , an implantation step (S 130 ) is performed. Specifically, an impurity of p type conductivity is implanted into breakdown voltage holding layer  122  using, as a mask, an oxide film formed through photolithography and etching, thereby forming p regions  123  as shown in  FIG. 11 . Further, after removing the oxide film thus used, an oxide film having a new pattern is formed through photolithography and etching. Using this oxide film as a mask, a conductive impurity of n type conductivity is implanted into predetermined regions to form n +  regions  124 . In a similar way, a conductive impurity of p type conductivity is implanted to form p +  regions  125 . As a result, the structure shown in  FIG. 11  is obtained. 
     After such an implantation step, an activation annealing process is performed. This activation annealing process can be performed under conditions that, for example, argon gas is employed as atmospheric gas, heating temperature is set at 1700° C., and heating time is set at 30 minutes. 
     Next, a gate insulating film forming step (S 140 ) is performed as shown in  FIG. 9 . Specifically, as shown in  FIG. 12 , oxide film  126  is formed to cover breakdown voltage holding layer  122 , p regions  123 , n +  regions  124 , and p +  regions  125 . As a condition for forming oxide film  126 , for example, dry oxidation (thermal oxidation) may be performed. The dry oxidation can be performed under conditions that the heating temperature is set at 1200° C. and the heating time is set at 30 minutes. 
     Thereafter, a nitrogen annealing step (S 150 ) is performed as shown in  FIG. 9 . Specifically, an annealing process is performed in atmospheric gas of nitrogen monoxide (NO). Temperature conditions for this annealing process are, for example, as follows: the heating temperature is 1100° C. and the heating time is 120 minutes. As a result, nitrogen atoms are introduced into a vicinity of the interface between oxide film  126  and each of breakdown voltage holding layer  122 , p regions  123 , n +  regions  124 , and p +  regions  125 , which are disposed below oxide film  126 . Further, after the annealing step using the atmospheric gas of nitrogen monoxide, additional annealing may be performed using argon (Ar) gas, which is an inert gas. Specifically, using the atmospheric gas of argon gas, the additional annealing may be performed under conditions that the heating temperature is set at 1100° C. and the heating time is set at 60 minutes. 
     Next, as shown in  FIG. 9 , an electrode forming step (S 160 ) is performed. Specifically, a resist film having a pattern is formed on oxide film  126  by means of the photolithography method. Using the resist film as a mask, portions of the oxide film above n +  regions  124  and p +  regions  125  are removed by etching. Thereafter, a conductive film such as a metal is formed on the resist film and formed in openings of oxide film  126  in contact with n +  regions  124  and p +  regions  125 . Thereafter, the resist film is removed, thus removing the conductive film&#39;s portions located on the resist film (lift-off). Here, as the conductor, nickel (Ni) can be used, for example. As a result, as shown in  FIG. 13 , source electrodes  111  and drain electrode  112  can be obtained. It should be noted that on this occasion, heat treatment for alloying is preferably performed. Specifically, using atmospheric gas of argon (Ar) gas, which is an inert gas, the heat treatment (alloying treatment) is performed with the heating temperature being set at 950° C. and the heating time being set at 2 minutes. 
     Thereafter, on source electrodes  111 , upper source electrodes  127  (see  FIG. 8 ) are formed. Further, drain electrode  112  is fowled on the backside surface of substrate  102  (see  FIG. 8 ). Further, gate electrode  110  (see  FIG. 8 ) is formed on oxide film  126 . In this way, semiconductor device  101  shown in  FIG. 8  can be obtained. Namely, semiconductor device  101  is fabricated by forming the epitaxial layer and the electrodes on SiC layer  20  of silicon carbide substrate  1 . 
     It should be noted that in the third embodiment, the vertical type MOSFET has been illustrated as one exemplary semiconductor device that can be fabricated using the silicon carbide substrate of the present invention, but the semiconductor device that can be fabricated is not limited to this. For example, various types of semiconductor devices can be fabricated using the silicon carbide substrate of the present invention, such as a JFET (Junction Field Effect Transistor), an IGBT (Insulated Gate Bipolar Transistor), and a Schottky barrier diode. Further, the third embodiment has illustrated a case where the semiconductor device is fabricated by forming the epitaxial layer, which serves as an active layer, on the silicon carbide substrate having its main surface corresponding to the (03-38) plane. However, the crystal plane that can be adopted for the main surface is not limited to this and any crystal plane suitable for the purpose of use and including the (0001) plane can be adopted for the main surface. 
     Example 
     The following describes an example of the present invention. An experiment was conducted to inspect electric characteristics in the intermediate layer (connection interface) of an actually fabricated silicon carbide substrate of the present invention. The experiment was conducted in the following manner. 
     First, a silicon carbide substrate of the present invention was fabricated as a sample. The silicon carbide substrate was fabricated in the same manner as in the first embodiment. Specifically, a base substrate and a SiC substrate were prepared. Employed as the base substrate was a substrate having a shape with a diameter Φ of 4 inches and a thickness of 300 μm, made of single-crystal silicon carbide with polytype of 4H, and having a main surface corresponding to the (03-38) plane. Further, the base substrate had n type conductivity, and had an n type impurity density of 1×10 20  cm −3 . Further, the base substrate had a micro pipe density of 1×10 4  cm −2 , and had a stacking fault density of 1×10 5  cm −1 . 
     Employed as the SiC substrate was a substrate having a planar shape of square with each side of 20 mm, having a thickness of 300 μm, made of single-crystal silicon carbide with a polytype of 4H, and having a main surface corresponding to the (03-38) plane. Further, the SiC substrate had n type conductivity, and had an n type impurity density of 1×10 19  cm −3 . Further, the SiC substrate had a micro pipe density of 0.2 cm −2  and had a stacking fault density less than 1 cm −1 . 
     Next, on the base substrate, a Si film having a thickness of 100 nm was formed using the sputtering method. Thereafter, the SiC substrate was placed on the Si film to fabricate a stacked substrate. Then, this stacked substrate was heated at 1500° C. for 3 hours, thereby converting at least portions of the Si film into silicon carbide to connect the base substrate and the SiC substrate to each other. The atmosphere during the heating was a mixed gas of hydrogen gas and propane, and has a pressure of 1×10 3  Pa. Further, the flow rate of the hydrogen gas was set at 3 μm, and the flow rate of propane was set at 80 sccm. It should be noted that the flow rate of the hydrogen gas can be set at 1 to 10 slm, and the flow rate of propane can be set at 50 to 500 sccm. With the above-described procedure, the silicon carbide substrate serving as the sample was fabricated. 
     Next, the main surface of the silicon carbide substrate obtained was polished to achieve a uniform thickness, whereby variation of the thickness (difference between the maximum value and the minimum value of the thickness of the silicon carbide substrate) became 5 μm. Further, ohmic electrodes were formed on both the main surfaces of the silicon carbide substrate. The ohmic electrodes were formed by forming nickel films on the main surfaces thereof and heating them for silicidation. The heat treatment for silicidation can be performed by heating them in an inert gas atmosphere to a temperature of not less than 900° C. and not more than 1100° C. for not less than 10 minutes and not more than 10 hours. In this experiment, the heat treatment was performed by heating them in an argon atmosphere under an atmospheric pressure to 1000° C. for 1 hour. Then, a voltage was applied between the ohmic electrodes to inspect electric characteristics of the connection interface (intermediate layer formed by converting at least portions of the Si film into silicon carbide). 
     As a result, it was confirmed that ohmic characteristics were obtained in the connection interface. From this, it was confirmed that according to the method for manufacturing the silicon carbide substrate of the present invention, the plurality of substrates made of silicon carbide can be connected to each other while securing ohmic characteristics in the thickness direction thereof. 
     The silicon carbide substrate of the present invention can be used to fabricate a semiconductor device as described above in the third embodiment. Namely, in the semiconductor device of the present invention, the epitaxial growth layer is formed as an active layer on the silicon carbide substrate manufactured using the method for manufacturing the silicon carbide substrate in the present invention. Explaining from a different point of view, in the semiconductor device of the present invention, the epitaxial growth layer is formed on the silicon carbide substrate of the present invention as an active layer. More specifically, the semiconductor device of the present invention includes: the silicon carbide substrate of the present invention; the epitaxial growth layer formed on the silicon carbide substrate; and the electrodes formed on the epitaxial growth layer. Namely, the semiconductor device of the present invention includes: the base layer made of silicon carbide; the intermediate layer fOrmed on and in contact with the base layer; the SiC layer made of single-crystal silicon carbide and disposed on and in contact with the intermediate layer; the epitaxial growth layer formed on the SiC layer; and the electrodes formed on the epitaxial growth layer. In addition, the intermediate layer contains silicon carbide at least at its region adjacent to the base layer and its region adjacent to the SiC layer, and connects the base layer and the SiC layer to each other. 
     The embodiments and example disclosed herein are illustrative and non-restrictive in any respect. The scope of the present invention is defined by the terms of the claims, rather than the embodiments described above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     INDUSTRIAL APPLICABILITY 
     A method for manufacturing a silicon carbide substrate, and the silicon carbide substrate in the present invention are particularly advantageously applicable to a method for manufacturing a silicon carbide substrate, and the silicon carbide substrate, each of which achieves reduced cost of manufacturing a semiconductor device using the silicon carbide substrate. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  2 : silicon carbide substrate;  10 : base layer (base substrate);  20 : SiC layer (SiC substrate);  20 A: main surface;  30 : Si film;  40 : intermediate layer;  101 : semiconductor device;  102 : substrate;  110 ; gate electrode;  111 : source electrode;  112 : drain electrode;  121 : buffer layer;  122 : breakdown voltage holding layer;  123 : p region;  124 : n +  region;  125 : p +  region;  126 : oxide film;  127 : upper source electrode.