1.  Field of the Invention
The present invention relates to a method of manufacturing a silicon carbide substrate, and a silicon carbide substrate. 
2.  Description of the Background Art
When the warp of a silicon carbide substrate is too great, inconvenience occurs in manufacturing a semiconductor device using the silicon carbide substrate.  Therefore, the level of warping must be regulated.  According to U.S. Pat.  No.  7,422,634, a value representing the warping of a wafer is represented by “warp” that is the distance between the maximum value and minimum value of the wafer surface measured from a reference plane.  This value is set smaller than approximately 0.5 μm.  The specification of the cited reference discloses a method of using a double-side lapping device as an exemplary method of manufacturing a wafer.  The publication teaches that polishing and etching are preferably carried out subsequent to usage of the double-side lapping device, and that the etching is dispensable. 
Assuming that an equivalent damage layer is formed at each of the top face and backside face of the wafer (silicon carbide substrate) subsequent to processing by the double-side lapping device, it is considered that the warp of the silicon carbide substrate may become smaller since the stress at both of the damage layers becomes equivalent.  However, the face formed by the lapping device generally has a relatively great surface roughness, not suitable to have an epitaxial layer of high quality formed thereon or a semiconductor element formed therefrom.  Therefore, polishing of further accuracy is required after lapping. 
If the aforementioned polishing is applied only to the top face (first face) among the top and backside faces, the damage layer at the top face and the damage layer at the backside face will no longer be equivalent since removal of the damage layer will be carried out at only the top face among the top face and backside face.  As a result, the warp of the silicon carbide substrate is increased.  This may cause inconvenience in manufacturing a semiconductor device.  For example, the resolution in photolithography is degraded, or the wafer may be cracked when being fastened. 
If the aforementioned polishing is carried out on both the top face and backside face, the surface roughness will be reduced, not only at the top face, but also at the backside face (second face).  In other words, both the top face and backside face are mirrored.  In other words, the appearance of each of the top face and backside face will become substantially the same.  As a result, distinguishing between the top face and backside face will become difficult.  Unlike a silicon substrate, the physical property differs between the top face and backside face of a silicon carbide substrate, since two types of planes, i.e. a Si plane and a C plane, are present due to the crystal structure thereof.  Therefore, distinguishing between the top face and backside face is important even if the surface roughness is the same.  Furthermore, the backside face having a mirrored face induces the problem that, when the backside face of the silicon carbide substrate is mounted on a stage, the backside face will slip on the stage or adhere to the stage.