Patent Publication Number: US-2023142939-A1

Title: MANUFACTURING METHOD OF SiC SUBSTRATE

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a manufacturing method of a silicon carbide (SiC) substrate. 
     Description of the Related Art 
     A power device such as an inverter or a converter is required to have large current capacity and high withstand voltage. To meet such requirements, power devices are often manufactured using a SiC substrate. Such a SiC substrate is typically manufactured from a SiC ingot. 
     For example, a SiC substrate is sliced from a SiC ingot with use of a wire saw or the like such that a silicon (Si)-face is exposed on a front surface and a carbon (C)-face is exposed on a back surface. It is to be noted that the Si-face is a surface terminated in Si and is expressed to be a (0001) plane by the Miller index notation. It is also to be noted that the C-face is a surface terminated in C and is expressed to be a (000-1) plane by the Miller index notation. 
     On the SiC substrate, the epitaxial growth of a SiC thin film on the Si-face is easier than that of a SiC thin film on the C-face. When manufacturing power devices with use of such a SiC substrate, the power devices are therefore typically formed on a side of a front surface on which a Si-face is exposed. 
     When an as-sliced SiC substrate (hereinafter simply called a “sliced SiC substrate”) is sliced from a SiC ingot, the sliced SiC substrate is however prone to be rough at its front surface and back surface (prone to have large irregularities formed at its front surface and back surface). If the front surface of the sliced SiC substrate is rough, it is difficult to allow an epitaxial growth of a SiC thin film on the front surface. There is hence a need to planarize (mirror-finish) the front surface of the sliced SiC substrate before power devices are manufactured using the sliced SiC substrate. 
     If the sliced SiC substrate is planarized only on its front surface, however, the resulting SiC substrate may significantly be warped due to a difference in roughness between its front surface and its back surface. A SiC substrate for use in the manufacture of power devices is hence manufactured by polishing and planarizing on the sides of both its front surface and back surface after grinding on both the sides of the front and back surfaces to reduce their roughness (see, for example, Japanese Patent Laid-open No. 2017-105697). 
     SUMMARY OF THE INVENTION 
     If power devices are formed on the side of only a front surface of a SiC substrate on which a Si-face is exposed, the planarization of the back surface of a sliced SiC substrate on which a C-face is exposed does not directly affect the performance of the power devices. On the contrary, the application of polishing not only to the side of a front surface but also to the side of the back surface of the sliced SiC substrate results in longer manufacturing lead time and also higher manufacturing cost. 
     With the foregoing in view, the present invention has as an object thereof the provision of a manufacturing method of a SiC substrate that can shorten the manufacturing lead time for the SiC substrate and can also reduce the manufacturing cost of the same. 
     In accordance with an aspect of the present invention, there is provided a manufacturing method of a SiC substrate. The manufacturing method includes a separation step of separating a sliced SiC substrate from a SiC ingot such that a Si-face is exposed on a front surface and a C-face is exposed on a back surface, a grinding step of, after the separation step, grinding the sliced SiC substrate on both a side of the front surface and a side of the back surface of the sliced SiC substrate, and a polishing step of, after the grinding step, polishing the sliced SiC substrate only on the side of the front surface and not on the side of the back surface. The grinding step includes a first grinding step of grinding the sliced SiC substrate on the side of the front surface and a second grinding step of grinding the sliced SiC substrate on the side of the back surface. In the second grinding step, the sliced SiC substrate is ground on the side of the back surface such that the back surface has an arithmetic mean height Sa of 1 nm or less. 
     Preferably, the second grinding step may be performed using grinding stones that contain abrasive grits having an average grit size of 0.3 μm or smaller. 
     In the present invention, the sliced SiC substrate is ground on the side of the front surface on which the Si-face is exposed, and is also ground on the side of the back surface on which the C-face is exposed, such that the back surface has the arithmetic mean height Sa of 1 nm or less, and the sliced SiC substrate is then polished only on the side of the front surface and not on the side of the back surface. If the sliced SiC substrate is ground on the side of the back surface as described above, the warpage of the resulting SiC substrate can be suppressed without the sliced SiC substrate being further polished on the side of the back surface. According to the present invention, it is hence possible to shorten the manufacturing lead time for a SiC substrate for use in the manufacture of power devices or the like, and to also reduce its manufacturing cost. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a flow chart schematically illustrating a manufacturing method according to an embodiment of the present invention for a SiC substrate; 
         FIG.  2    is a perspective view schematically illustrating an example of a sliced SiC substrate separated from a SiC ingot; 
         FIG.  3    is a perspective view schematically illustrating an example of a processing apparatus useful in the practice of the manufacturing method of  FIG.  1   ; 
         FIG.  4 A  is a side view schematically illustrating how the sliced SiC substrate is ground on a side of a front surface thereof; 
         FIG.  4 B  is a side view schematically illustrating how the sliced SiC substrate is ground on a side of a back surface thereof; and 
         FIG.  5    is a partly cross-sectional side view schematically illustrating how the sliced SiC substrate is polished on the side of the front surface thereof. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the attached drawings, a description will be made with regard to an embodiment of the present invention.  FIG.  1    is a flow chart schematically illustrating a manufacturing method according to the embodiment of the present invention for a SiC substrate. In this method, a sliced SiC substrate is separated from a cylindrical SiC ingot such that a Si-face is exposed on a front surface thereof and a C-face is exposed on a back surface thereof (separation step: S 1 ).  FIG.  2    is a perspective view schematically illustrating an example of the sliced substrate separated from the SiC ingot. 
     The sliced SiC substrate  11  illustrated in  FIG.  2    has been sliced from the SiC ingot such that the Si-face is exposed on its front surface  11   a  and the C-face is exposed on its back surface  11   b . This separation step (S 1 ) is performed by slicing of the sliced SiC substrate  11  from the SiC ingot, for example, with a wire saw such as a diamond wire saw. 
     As an alternative, the separation step (S 1 ) may also be performed by separation of the sliced SiC substrate  11  from the SiC ingot with a laser beam of a wavelength (for example, 1064 nm) that transmits SiC. If this is the case, the laser beam is first applied to the SiC ingot with a focal point of the laser beam positioned at a predetermined depth from a surface of the SiC ingot (a depth corresponding to a thickness of the sliced SiC substrate  11  to be separated). 
     As a consequence, a separation layer is formed inside the SiC ingot. An external force is then applied to the SiC ingot. As a result, the SiC ingot is split with the separation layer being used as a starting point of separation. In other words, the sliced SiC substrate  11  is separated from the SiC ingot. 
     After the sliced SiC substrate  11  has been ground on both the side of the front surface  11   a  and the side of the back surface  11   b  (grinding step: S 2 ), the sliced SiC substrate  11  is next polished only on the side of the front surface  11   a  and not on the side of the back surface  11   b  (polishing step: S 3 ).  FIG.  3    is a perspective view schematically illustrating an example of a processing apparatus that is useful in the practice of the manufacturing method of  FIG.  1   , specifically, that can grind and polish the sliced SiC substrate. 
     It is to be noted that an X-axis direction (front-to-rear direction) and a Y-axis direction (left-to-right direction) illustrated in  FIG.  3    are perpendicular to each other on a horizontal plane and that a Z-axis direction (up-to-down direction) is perpendicular (vertical) to the X-axis direction and Y-axis direction. 
     A processing apparatus  2  illustrated in  FIG.  3    includes a bed  4  that supports individual elements. A recessed section  4   a  is formed in an upper surface of the bed  4  at a location on a front side thereof, and in the recessed section  4   a , a transfer mechanism  6  is disposed to transfer the sliced SiC substrate  11  in a suction-held state. The transfer mechanism  6  can also reverse the sliced SiC substrate  11  upside down while holding the same. 
     In front of the recessed section  4   a , cassette tables  8   a  and  8   b  are disposed. Mounted on these cassette tables  8   a  and  8   b  are cassettes  10   a  and  10   b  which can each accommodate a plurality of sliced SiC substrates  11  or manufactured SiC substrates. Diagonally behind the recessed section  4   a , a position adjustment mechanism  12  is disposed to adjust the position of the sliced SiC substrate  11 . 
     This position adjustment mechanism  12  includes, for example, a table  12   a  configured to enable supporting of the sliced SiC substrate  11  at a central portion thereof and a plurality of pins  12   b  configured to be movable toward and away relative to the table  12   a  in a region outside the table  12   a . Loaded onto this table  12   a  is, for example, the sliced SiC substrate  11  unloaded from the cassette  10   a  by the transfer mechanism  6 . 
     At the position adjustment mechanism  12 , alignment is then performed for the sliced SiC substrate  11  loaded on the table  12   a . Described specifically, the pins  12   b  are brought toward the table  12   a  until they come into contact with a side surface of the sliced SiC substrate  11  loaded on the table  12   a , whereby the position of a center of the sliced SiC substrate  11  is aligned with a predetermined position on a plane (XY plane) parallel to the X-axis direction and Y-axis direction. 
     In the vicinity of the position adjustment mechanism  12 , a transfer mechanism  14  is disposed to swing such that the sliced SiC substrate  11  is transferred in a suction-held state. This transfer mechanism  14  includes a suction pad that can suction the sliced SiC substrate  11  on the side of an upper side thereof, and transfers the sliced SiC substrate  11  the position of which has been adjusted by the position adjustment mechanism  12 , rearward. Behind the transfer mechanism  14 , a disk-shaped turn table  16  is disposed. 
     This turn table  16  is connected to a rotary drive source (not illustrated) such as a motor, and rotates, using as an axis of rotation, a straight line that extends through a center of the turn table  16  and that is parallel to the Z-axis direction. On an upper surface of the turn table  16 , a plurality of (for example, four) chuck tables  18  are disposed at substantially equal intervals along a peripheral direction of the turn table  16 . 
     The transfer mechanism  14  then unloads the sliced SiC substrate  11  from the table  12   a  of the position adjustment mechanism  12 , and loads it onto the chuck table  18  placed at a loading and unloading position in the vicinity of the transfer mechanism  14 . The turn table  16  rotates, for example, in a direction indicated by an arrow in  FIG.  3   , and moves each chuck table  18  to the loading and unloading position, a coarse grinding position, a finish grinding position, and a polishing position in this order. 
     Each chuck table  18  is connected to a suction source (not illustrated) such as a vacuum pump, and can hold the sliced SiC substrate  11  that is placed on an upper surface of the chuck table  18 , by causing a suction force to act on the sliced SiC substrate  11 . Each chuck table  18  is also connected to a rotary drive source (not illustrated) such as a motor, and by power of the rotary drive source, can rotate using, as an axis of rotation, a straight line that extends through a center of the chuck table  18  and that is parallel to the Z-axis direction. 
     Behind the coarse grinding position and the finish grinding position (behind the turn table  16 ), columnar support structures  20  are disposed, respectively. On a front surface of each support structure  20  (a surface on a side of the turn table  16 ), a Z-axis moving mechanism  22  is disposed. This Z-axis moving mechanism  22  has a pair of guide rails  24  fixed on the front surface of the corresponding support structure  20  and extending along the Z-axis direction. 
     On a side of front surfaces of the paired guide rails  24 , a corresponding moving plate  26  is connected in a fashion that it is slidable along the paired guide rails  24 . Between the paired guide rails  24 , a corresponding screw shaft  28  extending along the Z-axis direction is disposed. To an upper end portion of the screw shaft  28 , a corresponding motor  30  is connected to rotate the screw shaft  28 . 
     On a surface of the screw shaft  28  in which a helical groove is formed, a nut portion (not illustrated) is disposed with numerous balls, which roll on the surface of the rotating screw shaft  28 , accommodated therein, so that a ball screw is configured. Rotation of the screw shaft  28  hence causes the numerous balls to circulate in the nut portion, whereby the nut portion moves along the Z-axis direction. 
     This nut portion is fixed on a side of a rear surface (back surface) of the moving plate  26 . When the screw shaft  28  is rotated by the motor  30 , the moving plate  26  thus moves together with the nut portion along the Z-axis direction. On a surface (front surface) of the moving plate  26 , a corresponding holder  32  is disposed. 
     This holder  32  supports a corresponding grinding unit  34  to grind the sliced SiC substrate  11 . The grinding unit  34  includes a corresponding spindle housing  36  fixed on the holder  32 . In this spindle housing  36 , a corresponding spindle  38  is accommodated in a rotatable fashion. The spindle  38  extends along the Z-axis direction. 
     To an upper end portion of each spindle  38 , a rotary drive source (not illustrated) such as a motor is connected. By power of this rotary drive source, the spindle  38  can rotate using, as an axis of rotation, a straight line that is parallel to the Z-axis direction. On the other hand, the spindle  38  is exposed at a lower end portion thereof from a lower surface of the spindle housing  36 , and a corresponding disk-shaped mount  40  is fixed on the lower end portion. 
     On a lower surface of the mount  40  of the grinding unit  34  on a side of the coarse grinding position, a coarse grinding wheel  42   a  is secured. This coarse grinding wheel  42   a  has a disk-shaped wheel base having substantially the same diameter as the mount  40 . On a lower surface of this wheel base, a plurality of grinding stones (coarse grinding stones) each having a parallelepiped shape are fixed. 
     Similarly, on a lower surface of the mount  40  of the grinding unit  34  on a side of the finish grinding position, a finish grinding wheel  42   b  is secured. This finish grinding wheel  42   b  includes a disk-shaped wheel base having substantially the same diameter as the mount  40 . On a lower surface of this wheel base, a plurality of grinding stones (finish grinding stones) each having a parallelepiped shape are fixed. 
     The coarse grinding stones and the finish grinding stones each contain abrasive grits made, for example, of diamond, cubic boron nitride (cBN), or the like and a binder that holds these abrasive grits. As the binder, a metal bond, a resin bond, a vitrified bond, or the like is used, for example. 
     It is to be noted that the abrasive grits contained in the finish grinding stones typically have a smaller average grit size than those contained in the coarse grinding stones. For example, the average grit size of the abrasive grits contained in the coarse grinding stones is 0.5 μm or greater but 30 μm or smaller, and the average grit size of the abrasive grits contained in the finish grinding stones is smaller than 0.5 μm. 
     In the vicinity of each of the grinding wheels  42   a  and  42   b , liquid supply nozzles (not illustrated) are arranged to supply liquid (grinding liquid) such as pure water to processing points to be used when the sliced SiC substrate  11  is to be ground. Alternatively, in place of or in addition to the nozzles, openings may be disposed in the grinding wheels  42   a  and  42   b  to supply the grinding liquid, and the grinding liquid may be supplied to processing points via the openings. 
     Beside the polishing position (beside the turn table  16 ), a support structure  44  is disposed. On a side surface of the support structure  44 , the side surface being on a side of the turn table  16 , an X-axis moving mechanism.  46  is disposed. This X-axis moving mechanism  46  has a pair of guide rails  48  fixed on the side surface of the support structure  44 , the side surface being on the side of the turn table  16 , and extending along the X-axis direction. 
     On surfaces of the paired guide rails  48 , the surfaces being on the side of the turn table  16 , a moving plate  50  is connected in a fashion that it is slidable along the paired guide rails  48 . Between the paired guide rails  48 , a screw shaft  52  extending along the X-axis direction is disposed. To a front end portion of the screw shaft  52 , a motor  54  is connected to rotate the screw shaft  52 . 
     On a surface of the screw shaft  52  in which a helical groove is formed, a nut portion (not illustrated) is disposed with numerous balls, which roll on the surface of the rotating screw shaft  52 , accommodated therein, so that a ball screw is configured. Rotation of the screw shaft  52  hence causes the numerous balls to circulate in the nut portion, whereby the nut portion moves along the X-axis direction. 
     This nut portion is fixed on a side of a surface (back surface) of the moving plate  50 , the surface (back surface) being opposite the support structure  44 . When the screw shaft  52  is rotated by the motor  54 , the moving plate  50  thus moves together with the nut portion along the X-axis direction. On a surface (front surface) of the moving plate  50 , the surface (front surface) being on the side of the turn table  16 , a Z-axis moving mechanism  56  is disposed. 
     This Z-axis moving mechanism  56  has a pair of guide rails  58  fixed on the front surface of the moving plate  50  and extending along the Z-axis direction. On surfaces of the paired guide rails  58 , the surfaces being on the side of the turn table  16 , a moving plate  60  is connected in a fashion that it is slidable along the paired guide rails  58 . 
     Between the paired guide rails  58 , a screw shaft  62  extending along the Z-axis direction is disposed. To an upper end portion of the screw shaft  62 , a motor  64  is connected to rotate the screw shaft  62 . On a surface of the screw shaft  62  in which a helical groove is formed, a nut portion (not illustrated) is disposed with numerous balls, which roll on the surface of the rotating screw shaft  62 , accommodated therein, so that a ball screw is configured. 
     Rotation of the screw shaft  62  hence causes the numerous balls to circulate in the nut portion, whereby the nut portion moves along the Z-axis direction. This nut portion is fixed on a side of a surface (rear surface) of the moving plate  60 , the surface (rear surface) being opposite the moving plate  50 . When the screw shaft  62  is rotated by the motor  64 , the moving plate  60  thus moves together with the nut portion along the Z-axis direction. 
     On a surface (front surface) of the moving plate  60 , the surface (front surface) being on the side of the turn table  16 , a holder  66  is disposed. This holder  66  supports a polishing unit  68  to polish the sliced SiC substrate  11 . The polishing unit  68  includes a spindle housing  70  fixed on the holder  66 . 
     In this spindle housing  70 , a spindle  72  is accommodated in a rotatable fashion. The spindle  72  extends along the Z-axis direction. To an upper end portion of the spindle  72 , a rotary drive source (not illustrated) such as a motor is connected. By power of this rotary drive source, the spindle  72  is rotated. 
     On the other nd, the spindle  72  is exposed at a lower end portion thereof from a lower surface of the spindle housing  70 , and a disk-shaped mount  74  is fixed on the lower end portion. On a lower surface of the mount  74 , a disk-shaped polishing pad  76  is secured. This polishing pad  76  has a disk-shaped base having substantially the same diameter as the mount  74 . 
     On a lower surface of this base, a disk-shaped polishing layer of substantially the same diameter as the mount  74  is fixed. This polishing layer is a fixed abrasive grit layer with abrasive grits dispersed thereinside. The polishing layer is produced, for example, by impregnation of a nonwoven fabric, which is made from a polyester, with a urethane solution in which abrasive grits of 0.4 to 0.6 μm average grit size are dispersed, and then drying of the impregnated nonwoven fabric. 
     The abrasive grits to be dispersed inside the polishing layer are formed of such a material as SiC, cBN, diamond, or fine metal oxide particles. As the fine metal oxide particles, fine particles made of silica (SiO 2 ), ceria (CeO 2 ), zirconia (ZrO 2 ), alumina (Al 2 O 3 ), or the like are used. The polishing layer is pliable, and slightly flexes according to pressures applied when the sliced SiC substrate  11  is being polished. 
     Radial center positions of the spindle  72 , the mount  74 , and the base and polishing layer of the polishing pad  76  substantially coincide together, and a cylindrical through-hole is formed to extend through these center positions. This through-hole is in communication with a polishing liquid supply source (not illustrated) that supplies liquid (polishing liquid) such as pure water to processing points to be used when the sliced SiC substrate  11  is to be polished. 
     This polishing liquid supply source has a reservoir, a supply pump, and the like for the polishing liquid. The polishing liquid supply source supplies the polishing liquid toward the chuck table  18  that is positioned at the polishing position, via the through-hole formed in the spindle  72  and the like. It is to be noted that abrasive grits may be contained or may not be contained in the polishing liquid. 
     Beside the transfer mechanism  14 , a transfer mechanism  78  is disposed to swing such that the sliced SiC substrate  11  is transferred in a suction-held state. This transfer mechanism  78  includes a suction pad that can suction the sliced SiC substrate  11  on the side of an upper side thereof, and transfers the sliced SiC substrate  11  that is placed on the chuck table  18  positioned at the loading and unloading position, forward. 
     In front of the transfer mechanism  78  and on a rear side of the recessed section  4   a , a rinsing system  80  is disposed. This rinsing system  80  is configured such that the sliced SiC substrate  11  that has been unloaded by the transfer mechanism  78  can be rinsed on the side of the upper side thereof. The sliced SiC substrate  11  rinsed by the rinsing system  80  is then transferred and placed, for example, in the cassette  10   b  by the transfer mechanism  6 . 
     On the processing apparatus  2 , the grinding step (S 2 ) and the polishing step (S 3 ) are performed, for example, in the following order. With the sliced SiC substrate  11  accommodated in the cassette  10   a  being suctioned on the side of the front surface  11   a , the transfer mechanism  6  first unloads the sliced SiC substrate  11  from the cassette  10   a , and loads the sliced SiC substrate  11  onto the table  12   a  of the position adjustment mechanism  12  such that the front surface  11   a  is directed upward. The pins  12   b  are then brought into contact with the sliced SiC substrate  11  such that alignment of the sliced SiC substrate  11  is performed. 
     With the thus-aligned sliced SiC substrate  11  being suctioned on the side of: the front surface  11   a , the transfer mechanism  14  next unloads the sliced SiC substrate  11  from the table  12   a , and loads it onto the chuck table  18  placed at the loading and unloading position such that the front surface  11   a  is directed upward. The chuck table  18  with the sliced SiC substrate  11  loaded thereon then holds under suction the sliced SiC substrate  11  on the side of the back surface (lower surface)  11   b . As illustrated in  FIG.  4 A , the sliced SiC substrate  11  is next polished on the side of the front surface  11   a.    
     Described specifically, the turn table  16  is first rotated such that the chuck table  18  with the sliced SiC substrate  11  held thereon is positioned at the coarse grinding position. While rotating both the chuck table  18  and the spindle  38  of the grinding unit  34  on the side of the coarse grinding position, the Z-axis moving mechanism  22  then lowers the grinding unit  34  on the side of the coarse grinding position such that the coarse grinding stones of the grinding wheel  42   a  and the front surface (upper surface)  11   a  of the sliced SiC substrate  11  are brought into contact with each other. 
     As a consequence, the sliced SiC substrate  11  is subjected to coarse grinding on the side of the front surface  11   a . During this time, the grinding liquid is supplied to contact interfaces (processing points) between the coarse grinding stones of the grinding wheel  42   a  and the front surface  11   a  of the sliced SiC substrate  11 . The rotational speeds of the chuck table  18  and the spindle  38  during this time are each, for example, 1000 rpm or higher but 5000 rpm or lower. Further, the lowering speed of the grinding unit  34  in the state in which the coarse grinding stones of the grinding wheel  42   a  and the front surface  11   a  of the sliced SiC substrate  11  are in contact with each other is, for example, 1 μm/sec or higher but 10 μm/sec or lower. 
     The Z-axis moving mechanism  22  next raises the grinding unit  34  on the side of the coarse grinding position such that the coarse grinding stones of the grinding wheel  42   a  and the front surface (upper surface)  11   a  of the sliced SiC substrate  11  separate from each other. The rotation of both the chuck table  18  and the spindle  38  of the grinding unit  34  on the side of the coarse grinding position is then stopped. The turn table  16  is then rotated such that the chuck table  18  with the sliced SiC substrate  11  held thereon is positioned at the finish grinding position. 
     While rotating both the chuck table  18  and the spindle  38  of the grinding unit  34  on the side of the finish grinding position, the Z-axis moving mechanism  22  then lowers the grinding unit  34  on the side of the finish grinding position such that the finish grinding stones of the grinding wheel  42   b  and the front surface (upper surface)  11   a  of the sliced SiC substrate  11  are brought into contact with each other. 
     As a consequence, the sliced SiC substrate  11  is subjected to finish grinding on the side of the front surface  11   a . During this time, the grinding liquid is supplied to contact interfaces (processing points) between the finish grinding stories of the grinding wheel  42   b  and the front surface  11   a  of the sliced SiC substrate  11 . The rotational speeds of the chuck table  18  and the spindle  38  during this time are each, for example, 1000 rpm or higher but 5000 rpm or lower. Further, the lowering speed of the grinding unit  34  in the state in which the finish grinding stones of the grinding wheel  42   b  and the front surface  11   a  of the sliced SiC substrate  11  are in contact with each other is, for example, lower than 1 μm/sec. 
     The Z-axis moving mechanism  22  next raises the grinding unit  34  on the side of the finish grinding position such that the finish grinding stones of the grinding wheel  42   b  and the front surface (upper surface)  11   a  of the sliced SiC substrate  11  separate from each other. The rotation of both the chuck table  18  and the spindle  38  of the grinding unit  34  on the side of the finish grinding position is then stopped. The grinding of the sliced SiC substrate  11  on the side of the front surface  11   a  (first grinding step) has now been completed. 
     The turn table  16  is next rotated such that the chuck table  18  with the sliced SiC substrate  11  held thereon passes the polishing position and is positioned at the loading and unloading position. The chuck table  18  positioned at the loading and unloading position is then caused to stop the suction of the sliced SiC substrate  11  on the side of the back surface (lower surface)  11   b.    
     In a state in which the sliced SiC substrate  11  placed on the chuck table  18  is suctioned on the side of the front surface (upper surface)  11   a , the transfer mechanism  78  next unloads the sliced SiC substrate  11  from the chuck table  18 , and loads it into the rinsing system  80  such that the front surface  11   a  is directed upward. The rinsing system  80  then rinses the sliced SiC substrate  11  on the side of the front surface  11   a.    
     With the sliced SiC substrate  11  suctioned on the side of the back surface  11   b , the transfer mechanism  6  next unloads the sliced SiC substrate  11  from the rinsing system  80 , and loads the sliced SiC substrate  11  onto the table  12   a  of the position adjustment mechanism  12  such that the back surface  11   b  is directed upward. The pins  12   b  are then brought into contact with the sliced SiC substrate  11  such that alignment of the sliced SiC substrate  11  is performed. 
     With the thus-aligned sliced SiC substrate  11  suctioned on the side of the back surface  11   b , the transfer mechanism  14  next unloads the sliced SiC substrate  11  from the table  12   a , and loads it onto the chuck table  18  placed at the loading and unloading position such that the back surface  11   b  is directed upward. The chuck table  18  with the sliced SiC substrate  11  loaded thereon then holds under suction the sliced SiC substrate  11  on the side of the front surface (lower surface)  11   a . As illustrated in  FIG.  4 B , the sliced SiC substrate  11  is next ground on the side of the back surface  11   b.    
     Described specifically, the turn table  16  is first rotated such that the chuck table  18  with the sliced SiC substrate  11  held thereon is positioned at the coarse grinding position. While rotating both the chuck table  18  and the spindle  38  of the grinding unit  34  on the side of the coarse grinding position, the Z-axis moving mechanism  22  then lowers the grinding unit  34  on the side of the coarse grinding position such that the coarse grinding stones of the grinding wheel  42   a  and the back surface (upper surface)  11   b  of the sliced SiC substrate  11  are brought into contact with each other. 
     As a consequence, the sliced SiC substrate  11  is subjected to coarse grinding on the side of the back surface  11   b . During this time, the grinding liquid is supplied to contact interfaces (processing points) between the coarse grinding stones of the grinding wheel  42   a  and the back surface  11   b  of the sliced SiC substrate  11 . The rotational speeds of the chuck table  18  and the spindle  38  during this time are each, for example, 1000 rpm or higher but 5000 rpm or lower. Further, the lowering speed of the grinding unit  34  in the state in which the coarse grinding stones of the grinding wheel  42   a  and the back surface  11   b  of the sliced SiC substrate  11  are in contact with each other is, for example, 1 μm/sec or higher but 10 μm/sec or lower. 
     The grinding wheel  42   a  at this time may be the same as that used when subjecting the sliced SiC substrate  11  to coarse grinding on the side of the front surface  11   a , or may be replaced to a different one. In other words, the coarse grinding stones for use in the coarse grinding of the sliced SiC substrate  11  on the side of the back surface  11   b  may be the same as those for use in the coarse grinding of the sliced SiC substrate  11  on the side of the front surface  11   a , or may be different ones. 
     The Z-axis moving mechanism  22  next raises the grinding unit  34  on the side of the coarse grinding position such that the coarse grinding stones of the grinding wheel  42   a  and the back surface (upper surface)  11   b  of the sliced SiC substrate  11  separate from each other. The rotation of both the chuck table  18  and the spindle  38  of the grinding unit  34  on the side of the coarse grinding position is then stopped. The turn table  16  is then rotated such that the chuck table  18  with the sliced SiC substrate  11  held thereon is positioned at the finish grinding position. 
     While rotating both the chuck table  18  and the spindle  38  of the grinding unit  34  on the side of the finish grinding position, the Z-axis moving mechanism  22  then lowers the grinding unit  34  on the side of the finish grinding position such that the finish grinding stones of the grinding wheel  42   b  and the back surface (upper surface)  11   b  of the sliced SiC substrate  11  are brought into contact with each other. 
     As a consequence, the sliced SiC substrate  11  is subjected to finish grinding on the side of the back surface  11   b . During this time, the grinding liquid is supplied to contact interfaces (processing points) between the finish grinding stones of the grinding wheel  42   b  and the back surface  11   b  of the sliced SiC substrate  11 . The rotational speeds of the chuck table  18  and the spindle  38  during this time are each, for example, 1000 rpm or higher but 5000 rpm or lower. Further, the lowering speed of the grinding unit  34  in the state in which the finish grinding stones of the grinding wheel  42   b  and the back surface  11   b  of the sliced SiC substrate  11  are in contact with each other is, for example, lower than 1 μm/sec. 
     The grinding wheel  42   b  at this time may be the same as that used when subjecting the sliced SiC substrate  11  to finish grinding on the side of the front surface  11   a , or may be replaced to a different one. In other words, the finish grinding stones for use in the finish grinding of the sliced SiC substrate  11  on the side of the back surface  11   b  may be the same as those for use in the finish grinding of the sliced SiC substrate  11  on the side of the front surface  11   a , or may be different ones. 
     The finish grinding of the sliced SiC substrate on the side of the back surface  11   b  is performed such that, after the finish grinding, the back surface  11   b  has an arithmetic mean height Sa of 1 nm or less. It is to be noted that an arithmetic mean height Sa is a parameter representing surface roughness as defined in ISO 25178, and is a parameter obtained by expanding an arithmetic mean height Ra, which is a parameter representing a line roughness, to a surface. 
     The Z-axis moving mechanism  22  next raises the grinding unit  34  on the side of the finish grinding position such that the finish grinding stones of the grinding wheel  42   b  and the back surface (upper surface)  11   b  of the sliced SiC substrate  11  separate from each other. The rotation of both the chuck table  18  and the spindle  38  of the grinding unit  34  on the side of the finish grinding position is then stopped. The grinding of the sliced SiC substrate  11  on the side of the back surface  11   b  (second grinding step) has now been completed. 
     The turn table  16  is next rotated such that the chuck table  18  with the sliced SiC substrate  11  held thereon passes the polishing position and is positioned at the loading and unloading position. The chuck table  18  positioned at the loading and unloading position is then caused to stop the suction of the sliced SiC substrate  11  on the side of the front surface (lower surface)  11   a.    
     In a state in which the sliced SiC substrate  11  placed on the chuck table  18  is suctioned on the side of the back surface (upper surface)  11   b , the transfer mechanism  78  next unloads the sliced SiC substrate  11  from the chuck table  18 , and loads it into the rinsing system  80  such that the back surface  11   b  is directed upward. The rinsing system  80  then rinses the sliced SiC substrate  11  on the side of the back surface  11   b.    
     With the sliced SiC substrate  11  suctioned on the side of the front surface  11   a , the transfer mechanism  6  next unloads the sliced SiC substrate  11  from the rinsing system  80 , and loads the sliced SiC substrate  11  onto the table  12   a  of the position adjustment mechanism  12  such that the front surface  11   a  is directed upward. The pins  12   b  are then brought into contact with the sliced SiC substrate  11  such that alignment of the sliced SiC substrate  11  is performed. 
     With the thus-aligned sliced SiC substrate  11  suctioned on the side of the front surface  11   a , the transfer mechanism  14  next unloads the sliced SiC substrate  11  from the table  12   a , and loads it onto the chuck table  18  placed at the loading and unloading position such that the front surface  11   a  is directed upward. The chuck table  18  with the sliced SiC substrate  11  loaded thereon then holds under suction the sliced SiC substrate  11  on the side of the back surface (lower surface)  11   b . As illustrated in  FIG.  5   , the sliced SiC substrate  11  is next polished on the side of the front surface  11   a.    
     Described specifically, the turn table  16  is first rotated such that the chuck table  18  with the sliced SiC substrate  11  held thereon passes the coarse grinding position and the finish grinding position and is positioned at the polishing position. While rotating both the chuck table  18  and the spindle  72  of the polishing unit  68 , the Z-axis moving mechanism  56  then lowers the polishing unit  68  such that the polishing layer of the polishing pad  76  and the front surface (upper surface)  11   a  of the sliced SiC substrate  11  are brought into contact with each other. 
     As a consequence, the sliced SiC substrate  11  is polished on the side of the front surface  11   a . During this time, a polishing liquid  13  is supplied from a polishing liquid supply source (not illustrated) to the front surface (upper surface)  11   a  of the sliced SiC substrate  11  via a through-hole  82  extending the spindle  72 , the mount  74 , and the polishing pad  76 . 
     The rotational speed of the chuck table  18  during this time is, for example, 300 rpm or higher but 750 rpm or lower. Further, the rotational speed of the spindle  72  during this time is, for example, 300 rpm or higher but 1000 rpm or lower. Meanwhile, a pressure applied to the front surface  11   a  of the sliced SiC substrate  11  during this time is, for example, 200 g/cm 2  or higher but 750 g/cm 2  or lower. 
     The Z-axis moving mechanism  56  next raises the polishing unit  68  such that the polishing layer of the polishing pad  76  and the front surface (upper surface)  11   a  of the sliced SiC substrate  11  separate from each other. The rotation of both the chuck table  18  and the spindle  72  is then stopped. The polishing of the sliced SiC substrate  11  on the side of the front surface  11   a  has now been completed. 
     The turn table  16  is next rotated such that the chuck table  18  with the sliced SiC substrate  11  held thereon is positioned at the loading and unloading position. The chuck table  18  positioned at the loading and unloading position is then caused to stop the suction of the sliced SiC substrate  11  on the side of the back surface (lower surface)  11   b.    
     In a state in which the sliced SiC substrate  11  placed on the chuck table  18  is suctioned on the side of the front surface (upper surface)  11   a , the transfer mechanism  78  next unloads the sliced SiC substrate  11  from the chuck table  18 , and loads it into the rinsing system  80  such that the front surface  11   a  is directed upward. The rinsing system  80  then rinses the sliced SiC substrate  11  on the side of the front surface  11   a.    
     With the resulting SiC substrate  11  (hereinafter simply called the “SiC substrate  11 ”) suctioned on the side of the front surface  11   a  or the back surface  11   b , the transfer mechanism  6  loads the SiC substrate  11  into the cassette  10   b . The grinding step (S 2 ) and the polishing step (S 3 ) on the processing apparatus  2  have now been completed. 
     In the above-mentioned manufacturing method of the SiC substrate, the sliced SiC substrate  11  is ground on the side of the front surface  11   a  on which the Si-face is exposed, and is ground on the side of the back surface  11   b  on which the C-face is exposed, such that the back surface  11   b  has an arithmetic mean height Sa of 1 nm or less, and the sliced SiC substrate  11  is then polished only on the side of the front surface  11   a  and not on the side of the back surface  11   b.    
     If the sliced SiC substrate  11  is ground on the side of the back surface  11   b  as described above, the warpage of the resulting SiC substrate  11  can be suppressed without the sliced SiC substrate  11  being further polished on the side of the back surface  11   b . According to this manufacturing method, it is therefore possible to shorten the manufacturing lead time for a SiC substrate for use in the manufacture of power devices or the like, and to also reduce its manufacturing cost. 
     It is to be noted that the above-mentioned method is an embodiment of the present invention and the present invention is hence not limited to the above-mentioned method. For example, the sliced SiC substrate  11  is ground on the side of the back surface  11   b  after being ground on the side of the front surface  11   a  in the grinding step (S 2 ) of the above-mentioned manufacturing method of the SiC substrate. In the grinding step (S 2 ) in the present invention, the sliced SiC substrate may however be ground on the side of the front surface  11   a  after being ground on the side of the back surface  11   b.    
     In this case, the sliced SiC substrate  11  held on the chuck table  18  can be polished on the side of the front surface  11   a  without reversing the sliced SiC substrate  11  upside down after the sliced substrate  11  is ground on the side of the front surface  11   a . If this is the case, it is therefore possible to further shorten the manufacturing lead time for a SiC substrate for use in the manufacture of power devices or the like, and to also further reduce its manufacturing cost. 
     It is also to be noted that the configurations, procedures, and the like of the above-mentioned embodiment can be practiced with appropriate changes or modifications within the scope not departing from the object of the present invention. 
     EXAMPLES 
     A description will hereinafter be made with regard to examples of the manufacturing method of the present invention for the SiC substrate. First, a cylindrical SiC ingot of 6 inches diameter was provided. With use of a diamond wire saw, three sliced SiC substrates were then sliced from the SiC ingot such that each sliced SiC substrate had a thickness of 500 to 600 μm and, in each sliced SiC substrate, an Si-face was exposed on a front surface and a C-face was exposed on a back surface. Coarse grinding and finish grinding were each applied to both the side of the front surface and the side of the back surface of one of the three sliced SiC substrates under the same conditions. 
     Described specifically, the coarse grinding was performed using a coarse grinding wheel having coarse grinding stones, which contained abrasive grits made of diamond of 14 μm average grit size, and a vitrified bond holding the abrasive grits. In that coarse grinding, the rotational speeds of the coarse grinding wheel and a chuck table with the sliced SiC substrate held thereon were each controlled at 2000 rpm. In the state in which the coarse grinding stones and the front surface or back surface of the sliced SiC substrate were in contact with each other, the lowering speed of a grinding unit was controlled at 3 μm/sec. 
     On the other hand, the finish grinding was performed using a finish grinding wheel having finish grinding stones, which contained abrasive grits made of diamond of 0.2 μm average grit size, and the vitrified bond holding the abrasive grits. In that finish grinding, the rotational speeds of the finish grinding wheel and a chuck table with the sliced SiC substrate held thereon were each controlled at 3000 rpm. In the state in which the finish grinding stones and the front surface or back surface of the sliced SiC substrate were in contact with each other, the lowering speed of a grinding unit was controlled at 0.15 μm/sec. As a result, a sliced SiC substrate of Example 1 (Ex. 1) was obtained. 
     Coarse grinding and finish grinding were next applied to the sides of both the surfaces of another one of the three sliced SiC substrates under the same conditions as those for the sliced SiC substrate in Example 1 except that the abrasive grits contained in the finish grinding stones that the finish grinding wheel had were different in average grit size. Described specifically, the finish grinding was performed using a finish grinding wheel having finish grinding stones, which contained abrasive grits made of diamond of 0.3 μm, and the vitrified bond holding the abrasive grits. As a result, a sliced SiC substrate of Example 2 (Ex. 2) was obtained. 
     Coarse grinding and finish grinding were next applied to the sides of both the surfaces of the remaining one of the three sliced SiC substrates under the same conditions as those for the sliced SiC substrates in Examples 1 and 2 except that the abrasive grits contained in the finish grinding stones that the finish grinding wheel had were different in average grit size. Described specifically, the finish grinding was performed using a finish grinding wheel having finish grinding stones, which contained abrasive grits made of diamond of 0.5 μm, and the vitrified bond holding the abrasive grits. As a result, a sliced SiC substrate of a comparative example (Comp. Ex.) was obtained. 
     The following Table 1 presents the arithmetic mean heights Sa of the back surfaces obtained after the finish grinding was applied to both the surfaces of the sliced SiC substrates in Examples 1 and 2 and the comparative example. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Ex. 1 
                 Ex. 2 
                 Comp. Ex. 
               
               
                   
                   
               
             
            
               
                   
                 Arithmetic mean 
                 0.59 
                 0.73 
                 1.88 
               
               
                   
                 height Sa (nm) 
               
               
                   
                   
               
            
           
         
       
     
     Polishing was next applied only to the sides of the front surfaces of the respective sliced SiC substrates of Examples 1 and 2 and the comparative example and not to the sides of their back surfaces. Described specifically, the polishing was performed using a polishing pad containing a polishing layer with abrasive grits made of SiO 2  of 0.4 to 0.6 μm grit size and dispersed in a nonwoven fabric. In the polishing, the rotational speed of the polishing pad was controlled at 745 rpm, the rotational speed of a chuck table with each SiC substrate held thereon was controlled at 750 rpm, and a pressure applied to the front surface of each sliced SiC substrate was controlled at 400 g/cm 2 . 
     The following Table 2 presents the amounts of warpage of the resulting SiC substrates obtained after the polishing was applied to only the sides of the front surfaces of the respective sliced SiC substrates of Examples 1 and 2 and the comparative example and not to the sides of their back surfaces. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Ex. 1 
                 Ex. 2 
                 Comp. Ex. 
               
               
                   
                   
               
             
            
               
                   
                 Amount of 
                 66.5 
                 109.1 
                 145.6 
               
               
                   
                 warpage of SiC 
                   
                   
                   
               
               
                   
                 substrate (μm) 
                   
                   
                   
               
               
                   
                   
               
            
           
         
       
     
     As presented in Tables 1 and 2, it has been found that grinding a sliced SiC substrate on the side of a front surface on which a Si-face is exposed and also grinding the sliced SiC substrate on the side of a back surface on which a C-face is exposed, such that the back surface has an arithmetic mean height Sa of 1 nm or less, enables the resulting SiC substrate to have a decreased amount of warpage even when polishing is applied to only the side of the front surface of the sliced SiC substrate and not to the side of its back surface. 
     The present invention is not limited to the details of the above described preferred embodiment. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.