Patent Publication Number: US-2023158640-A1

Title: Grinding wheel

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a grinding wheel for grinding a workpiece and a grinding method of grinding a workpiece with the grinding wheel. 
     Description of the Related Art 
     For thinning semiconductor device chips, in general, after devices such as integrated circuits (ICs) are formed on a front surface side of a wafer that is formed of a semiconductor such as silicon, a back surface side of the wafer is ground with a grinding apparatus. The grinding apparatus includes a disc-shaped chuck table that is rotatable about a predetermined rotational axis. There is disposed a grinding unit above the chuck table. The grinding unit has a spindle with a longitudinal portion substantially parallel to a Z-axis direction (for example, a vertical direction). 
     A lower end portion of the spindle has an annular-shaped grinding wheel mounted thereon with a disc-shaped mount interposed therebetween. The grinding wheel has an annular-shaped base made of metal. On one surface side (lower surface side) of the base, a plurality of grindstones each having abrasive grains and a bond are disposed at substantially equal intervals along a circumferential direction of the base. When the back surface side of the wafer is ground by infeed grinding, the chuck table that holds under suction the front surface side of the wafer is rotated about a predetermined rotational axis. In addition, while supplying grinding water such as pure water onto the back surface side of the wafer, at the same time, the grinding unit is fed for processing at a predetermined speed, with the spindle rotating the grinding wheel (for example, see Japanese Patent Laid-Open No. 2014-124690). 
     Meanwhile, a to-be-ground surface of the wafer may be formed with a layer or a film made of a material or a quality different from a material of the wafer. For example, a to-be-ground surface of a silicon carbide (SiC) wafer may be formed with a modified layer in which silicon (Si) and carbon (C) are decomposed. As another alternative, a to-be-ground surface of a silicon wafer may be formed with a silicon oxide film. In this manner, in a case in which a layer or a film made of a material or a quality different from a material of the wafer is formed on the to-be-ground surface of the wafer, in general, a first grinding wheel is used in order to grind the layer or the film described above, and then, after the layer or the film is removed, the second grinding wheel having a characteristic different from that of a first grinding wheel is used in order to grind the wafer. 
     However, in general, since one grinding unit (in other words, one spindle) has one grinding wheel mounted thereon, two grinding units are required for grinding the wafer by use of the first grinding wheel and the second grinding wheel which have grinding characteristics different from each other. 
     SUMMARY OF THE INVENTION 
     However, in a case in which the two grinding apparatuses each having one grinding unit are used to grind a wafer, a step of transferring the wafer between the grinding apparatuses is additionally required, resulting in an increased number of manufacturing steps. Moreover, if two grinding apparatuses are used as described above, footprint for apparatuses in such a room as a clean room is increased. The present invention is achieved in view of these matters, and it is an object of the present invention to perform grinding on a wafer by mounting one grinding wheel on one spindle, the grinding exercising grinding similar to grinding performed by use of two grinding wheels having grinding characteristics different from each other. 
     In accordance with an aspect of the present invention, there is provided a grinding wheel that is mounted on a tip end of a spindle and grinds a workpiece according to rotation of the spindle, the grinding wheel including an annular base, and a plurality of grindstone sets disposed in an annular manner on one surface side of the base along a circumferential direction of the base. Each of the plurality of grindstone sets has a first grindstone and a second grindstone having a self-sharpening capability lower than that of the first grindstone, such that the first grindstone and the second grindstone are adjacent to each other in a predetermined orientation in the circumferential direction of the base, and a predetermined interval larger than an interval between the first grindstone and the second grindstone in the circumferential direction of the base is provided between adjacent ones of the grindstone sets. 
     In accordance with another aspect of the present invention, there is provided a grinding method of grinding a workpiece having a single crystal substrate formed with a non-single crystal layer on a front surface of the substrate, with use of the grinding wheel described above. The grinding method includes a holding step of holding under suction, on a chuck table, a back surface of the substrate that is positioned opposite to the front surface of the substrate, a first grinding step of rotating the spindle in a predetermined direction such that the second grindstone is set in front of the first grindstone in each of the grindstone sets, to grind the front surface of the substrate, thereby removing the non-single crystal layer, after the holding step is carried out, and a second grinding step of rotating the spindle in a direction opposite to the predetermined direction such that the first grindstone is set in front of the second grindstone in each of the grindstone sets, to grind the substrate, after the first grinding step is carried out. 
     In accordance with a further aspect of the present invention, there is provided a grinding method of grinding a workpiece having a substrate formed with a film on a front surface of the substrate, with use of the grinding wheel described above. The grinding method includes a holding step of holding under suction, on a chuck table, a back surface of the substrate that is positioned opposite to the front surface of the substrate, a first grinding step of rotating the spindle in a predetermined direction such that the first grindstone is set in front of the second grindstone in each of the grindstone sets, to grind the film, thereby removing the film, after the holding step is carried out, and a second grinding step of rotating the spindle in a direction opposite to the predetermined direction such that the second grindstone is set in front of the first grindstone in each of the grindstone sets, to grind the substrate, after the first grinding step is carried out. 
     With use of the grinding wheel according to an embodiment of the present invention, the grinding characteristic can be changed between grinding in which the spindle is rotated in a predetermined direction such that the second grindstone is set in front of the first grindstone in each of the grindstone sets, and grinding in which the spindle is rotated in a direction opposite to the predetermined direction such that the first grindstone is set in front of the second grindstone in each of the grindstone sets. In other words, changing the rotational direction of one spindle makes it possible to grind the workpiece with a different grinding characteristic. Hence, with one grinding wheel mounted on one spindle, grinding similar to grinding performed by use of two grinding wheels having the grinding characteristics different from each other can be performed on the workpiece. 
     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 some preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a top view of a grinding wheel according to a first embodiment of the present invention; 
         FIG.  1 B  is a cross-sectional view of the grinding wheel of  FIG.  1 A ; 
         FIG.  1 C  is a bottom view of the grinding wheel of  FIG.  1 A ; 
         FIG.  2    is a perspective view of a grinding apparatus; 
         FIG.  3    is a top view illustrating a chuck table and other constituent elements; 
         FIG.  4    is a flow chart of a grinding method according to the first embodiment of the present invention; 
         FIG.  5    is a diagram illustrating a holding step of the grinding method of  FIG.  4   ; 
         FIG.  6 A  is a diagram illustrating a first grinding step of the grinding method of  FIG.  4   ; 
         FIG.  6 B  is a cross-sectional view of a partly enlarged portion of  FIG.  6 A , in the first grinding step; 
         FIG.  7 A  is a diagram illustrating a second grinding step of the grinding method of  FIG.  4   ; 
         FIG.  7 B  is a cross-sectional view of a partly enlarged portion of  FIG.  7 A , in the second grinding step; 
         FIG.  8    is a diagram illustrating a holding step according to a grinding method of a second embodiment of the present invention; 
         FIG.  9    is a diagram illustrating a first grinding step of the grinding method according to the second embodiment; 
         FIG.  10    is a diagram illustrating a second grinding step of the grinding method according to the second embodiment; 
         FIG.  11 A  is a diagram illustrating a correspondence relation between a rotational direction and a processing region of a chuck table, in a case in which a second grindstone is set in front of a first grindstone; and 
         FIG.  11 B  is a diagram illustrating a correspondence relation between the rotational direction and the processing region of the chuck table, in a case in which the first grindstone is set in front of the second grindstone. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.  FIG.  1 A  is a top view of a grinding wheel  2 .  FIG.  1 B  is a cross-sectional view of the grinding wheel  2 .  FIG.  1 C  is a bottom view of the grinding wheel  2 . The grinding wheel  2  has an annular base  4  formed of metal such as an aluminum alloy. For example, an outer diameter of the base  4  is 200 mm, but the outer diameter of the base  4  is appropriately selectable according to a purpose of grinding, for example. 
     The base  4  has an upper surface (other surface)  4   a  and a lower surface (one surface)  4   b , and each surface is substantially flat, having an annular shape. The upper surface  4   a  has a plurality of threaded holes  4   c  formed therein at substantially equal intervals along a circumferential direction of the base  4 , and the threaded holes  4   c  are used at a time of mounting the grinding wheel  2  on a mount  28  illustrated in  FIG.  2   . Note that an inner side surface in a truncated conical shape of the base  4  located on the inner peripheral side relative to the lower surface  4   b  may be provided with a plurality of openings (not illustrated) each for supplying grinding water such as pure water, along the circumferential direction of the base  4  at substantially equal intervals. 
     As illustrated in  FIG.  1 C , on the lower surface  4   b  of the base  4 , a plurality of grindstone sets  6  are disposed in an annular manner along the circumferential direction of the base  4  at substantially equal intervals. Although  24  grindstone sets  6  are disposed in  FIG.  1 C , the number of grindstone sets  6  is not limited to  24 . As illustrated in  FIG.  1 B , one grindstone set  6  has a first grindstone  8  and a second grindstone  10 , each grindstone having a segment shape. An upper end portion of each of the first grindstone  8  and the second grindstone  10  is disposed in a groove portion formed on the lower surface  4   b  side of the base  4  and is fixed to the base  4  via an adhesive (not illustrated). In addition, each of the first grindstone  8  and the second grindstone  10  has a lower end portion protruding from the lower surface  4   b  in such a manner that the lower end portion is equivalent to a predetermined protruding amount (also referred to as a segment height). The protruding amounts of the first grindstone  8  and the second grindstone  10  are substantially the same. 
     The first grindstones  8  and the second grindstones  10  are formed of the same materials and have abrasive grains with the same grain size (for example, diamond abrasive grains), and concentration of the abrasive grains is substantially same in the first grindstone  8  and the second grindstone  10 , for example, while a material of a bond and porosity are different between the first grindstone  8  and the second grindstone  10 . In the present specification, the first grindstone  8  is relatively soft, and the second grindstone  10  is harder than the first grindstone  8 . A hardness of the grindstones can be evaluated by a three-point bend test, for example. A bending strength of the second grindstone  10  is two or more times greater than that of the first grindstone  8 . 
     A hardness of the grindstone is attributable to a material of the bond, for example. In a case in which the bond of the first grindstone  8  is a resin bond and the bond of the second grindstone  10  is a vitrified bond, a hardness of the second grindstone  10  is greater than that of the first grindstone  8 . In addition, for example, the hardness of the grindstone is attributable to its porosity. In a case in which the bond of the first grindstone  8  and the bond of the second grindstone  10  are both vitrified bonds of substantially the same components and porosity of the first grindstone  8  is higher than that of the second grindstone  10 , the hardness of the second grindstone  10  is greater than that of the first grindstone  8 . 
     Moreover, similarly, in a case in which the bond of the first grindstone  8  and the bond of the second grindstone  10  are both resin bonds of substantially the same components and the porosity of the first grindstone  8  is higher than porosity of the second grindstone  10 , the hardness of the second grindstone  10  is greater than that of the first grindstone  8 . In this manner, in a case in which the hardness of the second grindstone  10  is greater than that of the first grindstone  8 , a force of fixing the abrasive grains by, for example, the bond of the second grindstone  10  (i.e., a bonding force) is greater than the bonding force of the bond of the first grindstone  8 . Due to a difference between these bonding forces described above, a self-sharpening capability of the first grindstone  8  that is relatively soft is relatively high, and a self-sharpening capability of the second grindstone  10  that is relatively hard is lower than that of the first grindstone  8 . 
     In one grindstone set  6 , the first grindstone  8  and the second grindstone  10  are disposed in the circumferential direction of the base  4  so as to be oriented in a predetermined direction. In the bottom view illustrated in  FIG.  1 C , in one grindstone set  6 , the first grindstone  8  and the second grindstone  10  are disposed in such a manner that an orientation advancing from the second grindstone  10  to the first grindstone  8  is set clockwise. However, in a case in which the grinding wheel  2  is similarly viewed from the bottom of the grinding wheel  2 , in one grindstone set  6 , the first grindstone  8  and the second grindstone  10  may be disposed in such a manner that an orientation advancing from the first grindstone  8  to the second grindstone  10  is set clockwise. 
     As illustrated in  FIG.  1 C , in one grindstone set  6 , the first grindstone  8  and the second grindstone  10  are provided so as to be adjacent to each other. The first grindstone  8  and the second grindstone  10  of the present embodiment are substantially in contact with each other; however, they may be adjacent to each other to such an extent that there can be provided a small gap between the first grindstone  8  and the second grindstone  10  in the circumferential direction of the base  4 . In this case, the small gap between the first grindstone  8  and the second grindstone  10  in the circumferential direction of the base  4  is sufficiently smaller than a predetermined interval  6   a  between two grindstone sets  6  adjacent to each other in the circumferential direction of the base  4 , for example. 
     The interval between the first grindstone  8  and the second grindstone  10  in the circumferential direction of the base  4  is ⅓ or smaller than the predetermined interval  6   a , preferably ¼ or smaller than the predetermined interval  6   a . For example, the interval between the first grindstone  8  and the second grindstone  10  in the circumferential direction of the base  4  is 1 mm or smaller. In addition, for example, the first grindstone  8  and the second grindstone  10  are close to each other to such an extent that the abrasive grains protruding from a side surface of the first grindstone  8  adjacent to the second grindstone  10  are in contact with the second grindstone  10  or that the abrasive grains protruding from a side surface of the second grindstone  10  adjacent to the first grindstone  8  are in contact with the first grindstone  8 . 
     As described above, the predetermined interval  6   a  is provided between adjacent ones of the grindstone sets  6 , and the predetermined interval  6   a  is larger than the small gap between the first grindstone  8  and the second grindstone  10  in the circumferential direction of the base  4 . The predetermined interval  6   a  along the circumferential direction of the base  4  is a predetermined value of 3.0 mm or more but 4.0 mm or less, for example, and the predetermined interval  6   a  is appropriately adjusted, depending on the numbers of the first grindstone  8  and the second grindstone  10 , each size of the first grindstone  8  and the second grindstone  10 , and the like. 
     The first grindstones  8  and the second grindstones  10  are formed separately after the bond, the abrasive grains, a filler, and the like are first mixed and the mixture is subsequently subjected to compression molding, baking, and shaping, for example. As the abrasive grains, what is generally called superabrasive grains such as diamond or cubic boron nitride (cBN) are used. After shaping, in such a manner as to achieve the layout illustrated in  FIG.  1 A  to  FIG.  1 C , the first grindstones  8  and the second grindstones  10  are fixed to the base  4  via the adhesive. The grinding wheel  2  manufactured in this manner is used in a grinding apparatus  12  which grinds a workpiece  11  (see  FIG.  5    or the like). 
       FIG.  2    is a perspective view of the grinding apparatus  12 . Note that  FIG.  2    illustrates a main portion of the grinding apparatus  12  and for the convenience of description, illustration of other constituent elements is omitted. In addition, a Z-axis direction illustrated in  FIG.  2    is parallel to a vertical direction, for example. The grinding apparatus  12  has a circular disc-shaped chuck table  14 . The chuck table  14  has a circular disc-shaped frame member  16  (see  FIG.  5   ) that is formed of non-porous ceramics. An upper portion of the frame member  16  has a circular disc-shaped recess formed therein, and this recess has a circular disc-shaped porous plate  18  (see  FIG.  5   ) formed of porous ceramics fixed thereto. 
     The frame member  16  has a gas flow path  16   a  (see  FIG.  5   ) formed therein, and the gas flow path  16   a  has one end portion connected to a suction source such as an ejector (not illustrated). A negative pressure from the suction source is transmitted through the gas flow path  16   a  to the porous plate  18 . An upper surface of the frame member  16  and an upper surface of the porous plate  18  are substantially flush with each other, and they function as a holding surface  14   a  for holding under suction the workpiece  11 . A central portion of the porous plate  18  protrudes more than the peripheral portion, and the holding surface  14   a  is a conical shape. However, a protruding amount of the central portion of the porous plate  18  is much smaller than a diameter of the holding surface  14   a . For example, whereas the protruding amount of the central portion of the porous plate  18  is 20 µm, the diameter of the holding surface  14   a  is 200 mm. In view of this, in  FIG.  5   , for the convenience of description, the holding surface  14   a  is illustrated to be substantially flat. 
     A lower portion of the chuck table  14  has a rotational axis  20  (see  FIG.  5   ) provided therein, the rotational axis  20  being cylindrical. The rotational axis  20  is rotated in a predetermined direction by a driving mechanism such as a pulley and a belt (not illustrated). In  FIG.  2   , a rotational center  20   a  of the rotational axis  20  is indicated with a one-dot chain line. The chuck table  14  is supported by a table base  22  in a rotatable manner. A lower portion of the table base  22  has an inclination adjusting mechanism (not illustrated) provided therein, the inclination adjusting mechanism for adjusting inclination of the table base  22 . 
     The inclination adjusting mechanism has a first supporting portion, a second supporting portion, and a third supporting portion. A length of the first supporting portion in the Z-axis direction is fixed, while lengths of the second supporting portion and the third supporting portion in the Z-axis direction can separately be changed. The first to third supporting portions are disposed at substantially equal intervals in a circumferential direction of the table base  22  to support the table base  22  in three points. 
     On the periphery of the chuck table  14 , an inner nozzle (not illustrated) for supplying grinding water onto a contact region (processing region) between the grinding wheel  2  and the workpiece is provided. In the present embodiment, inner edge grinding in which the workpiece is ground on an inner circumferential side of the grindstones is performed, and accordingly, an inner nozzle for supplying the grinding water on the inner circumferential side of the grindstones is used. Alternatively, the grinding water may be supplied from the openings provided in the base  4 . Note that, in a case in which an outer edge grinding in which the workpiece is ground on an outer circumferential side of the grindstones is performed, an outer nozzle (not illustrated) for supplying the grinding water on the outer circumferential side of the grindstones may be used. 
     Above the chuck table  14 , there is provided a grinding unit  24 . The grinding unit  24  has a cylindrical spindle housing (not illustrated). The spindle housing has a Z-axis direction moving unit of ball-screw type (not illustrated) mounted thereto. The Z-axis direction moving unit can vertically move the spindle housing in the Z-axis direction. For example, at a time of grinding, the Z-axis direction moving unit moves the grinding unit  24  downward (i.e., processing-feeds) at a predetermined processing-feeding speed. 
     The spindle housing houses part of a cylindrical spindle  26  in a rotatable manner. On the periphery of an upper end portion of the spindle housing, there is provided a driving source (not illustrated), such as a motor, for rotating the spindle  26 . According to an orientation of current supplied to the driving source, the spindle  26  can rotate in both clockwise and counterclockwise directions. A lower end portion (tip end portion)  26   a  of the spindle  26  protrudes from the spindle housing. 
     The lower end portion  26   a  has an upper surface of a circular disc-shaped mount  28  fixed thereto. On a lower surface side of the mount  28 , the grinding wheel  2  described above is mounted with a plurality of bolts  30 . In this manner, the lower end portion  26   a  has the grinding wheel  2  mounted thereto with the mount  28  interposed therebetween. As a result of rotation of the spindle  26 , the grinding wheel  2  rotates, and accordingly, a rotational trajectory of the lower surface of each of the first grindstone  8  and the second grindstone  10  forms an annular grinding surface. 
     The rotational axis  20  of the chuck table  14  is inclined in such a manner that a partial region  14   b  of the holding surface  14   a  (see  FIG.  3   ) is substantially parallel to the grinding surface. The partial region  14   b  is an arc-shaped region passing a center of the holding surface  14   a . The partial region  14   b  is located directly below the plurality of grindstone sets  6 , and in  FIG.  3   , a range of the partial region  14   b  is indicated with a double-headed arrow. At a time of grinding, of the workpiece  11  held under suction on the holding surface  14   a , the region corresponding to this partial region  14   b  is ground (i.e., becomes a processing region).  FIG.  3    is a top view of the chuck table  14  and other constituent elements. Note that, in  FIG.  3   , only the plurality of grindstone sets  6  of the grinding unit  24  located on the holding surface  14   a  are indicated in addition to the chuck table  14 . 
     Next, with reference to  FIG.  4    to  FIG.  7 B , a grinding method of the first embodiment of the present invention will be described.  FIG.  4    is a flow chart of a grinding method according to the first embodiment of the present invention. In the first embodiment, in the order of a holding step S 10 , a first grinding step S 20 , and a second grinding step S 30 , infeed grinding is carried out on the workpiece  11 . The workpiece  11  has a circular disc shape, and has a single crystal substrate (the wafer)  13  mainly formed of silicon carbide (SiC). 
     When the workpiece  11  is formed, first, a laser beam is focused on an SiC ingot at a predetermined depth to form an amorphous region (also referred to as a modified region, a separation region, or the like). In the amorphous region, amorphous carbon (C) and amorphous silicon (Si) are considered to be formed. After the amorphous region is formed, a substrate  13  is separated from the ingot with the amorphous region as a boundary. As a result, a front surface  13   a  of the substrate  13  separated from the ingot has an amorphous layer (non-single crystal layer)  15  formed therein. Note that the front surface  13   a  of the substrate  13  having been separated has an arithmetic mean roughness Ra of substantially 80 µm to 100 µm, for example. 
     When the workpiece  11  is ground, first, a back surface  13   b  side of the substrate  13  located on an opposite side of the front surface  13   a  is held under suction on the holding surface  14   a  of the chuck table  14  (holding step S 10 ).  FIG.  5    is a diagram illustrating the holding step S 10 . Subsequent to the holding step S 10 , the first grinding step S 20  is carried out.  FIG.  6 A  is a diagram illustrating the first grinding step S 20 . In the first grinding step S 20 , to remove the amorphous layer  15 , mainly with use of the second grindstone  10  being relatively hard and having a low self-sharpening capability, the front surface  13   a  side of the substrate  13  is ground. 
     In the first grinding step S 20 , the chuck table  14  is rotated at a predetermined value of 100 rpm or higher but 300 rpm or less in a predetermined direction, and the spindle  26  is rotated at a predetermined value of 1000 rpm or higher but 4000 rpm or less in a predetermined direction. At the same time, while supplying the grinding water to a processing point along with these rotational operations of the chuck table  14  and the spindle  26 , the grinding unit  24  is fed for grinding at a predetermined processing feeding speed of 0.1 µm/s or higher but 2.0 µm/s or less. Particularly, in the first grinding step S 20 , the spindle  26  and the grinding wheel  2  are rotated in a predetermined direction in such a manner that the second grindstone  10  is set in front of the first grindstone  8  in each of the grindstone sets  6 . 
       FIG.  6 B  is a cross-sectional view of a partly enlarged portion of  FIG.  6 A , in the first grinding step S 20 . As indicated in  FIG.  6 B , in the first grinding step S 20 , mainly with use of the second grindstone  10  of each of the grindstone sets  6 , the front surface  13   a  side of the substrate  13  is ground. Accordingly, it is possible to carry out grinding on which a grinding characteristic of the second grindstone  10  is reflected. Specifically, it is possible to grind the front surface  13   a  side of the substrate  13 , while achieving reduction in wear amount of the grindstone. Note that, in the first grinding step S 20 , the first grindstone  8  being relatively soft and having a high self-sharpening capability only traces the front surface  13   a  having ground by the second grindstone  10 . As a result, the wear amount of the first grindstone  8  is substantially the same as the wear amount of the second grindstone  10 . In this manner, in the first grinding step S 20 , it is possible to grind the workpiece  11 , relatively reducing the wear amounts of the grindstones. In the first grinding step S 20 , the front surface  13   a  side of the substrate  13  is ground until the amorphous layer  15  is removed and the substrate  13  is then exposed. 
     Subsequent to the first grinding step S 20 , the second grinding step S 30  is carried out. The single crystal substrate  13  is harder than the region of the amorphous layer  15 . When this hard substrate  13  is ground, glazing and shedding of the grindstones are likely to occur. To address this problem, in order to grind the substrate  13  stably, there is a need to proceed grinding while promoting self-sharpening of the grindstones. In the second grinding step S 30 , the first grindstone  8  being relatively soft and having a high self-sharpening capability is effectively used to grind the substrate  13 . 
     First, in the second grinding step S 30 , the grinding unit  24  is raised to such an extent that the first grindstone  8  and the second grindstone  10  are not in contact with the workpiece  11 . Note that a rotational direction and a rotational speed of the chuck table  14  are maintained at the same rotational direction and the same rotational speed in the first grinding step S 20 . Then, in such a manner that the first grindstone  8  is set in front of the second grindstone  10  in each of the grindstone sets  6 , the spindle  26  and the grinding wheel  2  are rotated at a predetermined value of 1000 rpm or higher but 4000 rpm or less in a direction opposite to the predetermined direction in the first grinding step S 20 . Subsequently, while rotating the spindle  26  and supplying the grinding water to the processing point, the grinding unit  24  is fed for grinding at a processing feeding speed of a predetermined value of 0.1 µm/s or higher but 2.0 µm/s or less. 
       FIG.  7 A  is a diagram illustrating the second grinding step S 30  of the grinding method of  FIG.  4   . In the second grinding step S 30 , it is possible to perform grinding on which a grinding characteristic of the first grindstone  8  is reflected. Specifically, it is possible to perform grinding on which a high self-sharpening capability is reflected. In addition, some abrasive grains  8   b  falling off a bond  8   a  of the first grindstone  8  enter the lower surface (grinding surface) of the second grindstone  10 , so that self-sharpening of the second grindstone  10  being relatively hard and having a low self-sharpening capability is promoted. 
       FIG.  7 B  is a cross-sectional view of a partly enlarged portion of  FIG.  7 A , in the second grinding step S 30 . As indicated in  FIG.  7 B , in the second grinding step S 30 , some abrasive grains  8   b  falling off can promote self-sharpening of the second grindstone  10 . As a result, the first grindstone  8  and the second grindstone  10  are substantially the same in wear amount with each other. In this manner, in the second grinding step S 30 , while each wear amount of the first grindstone  8  and the second grindstone  10  is increased more than that in the first grinding step S 20 , the workpiece  11  is ground to grind the substrate  13  until the substrate  13  has a predetermined finished thickness. 
     In the first embodiment, with use of one grinding wheel  2  mounted on one spindle  26 , according to the rotational direction of the spindle  26 , grinding is achieved such that the wear amounts of the grindstones are reduced in the first grinding step S 20 . In addition, in the second grinding step S 30 , it is possible to exercise a high self-sharpening capability. In this manner, it is possible to change a grinding characteristic according to a rotational direction of the spindle  26 . In other words, according to a rotational direction of the spindle  26 , it is possible to perform grinding, on the workpiece  11 , similar to grinding performed by use of two grinding wheels having grinding characteristics different from each other. Hence, it is possible to prevent an additional transferring step between the grinding apparatuses and an increase in footprint of the grinding apparatuses. Note that, since the plurality of grindstone sets  6  are disposed in an annular manner along the circumferential direction of the base  4 , regardless of the rotational direction of the spindle  26 , an inner diameter and an outer diameter of the grinding surface of the grinding wheel  2  are allowed to be substantially the same. 
     Meanwhile, according to an experiment having conducted by the present applicant, in a case in which the wear amount of each of the first grindstone  8  and the second grindstone  10  in the first grinding step S 20  is set to  1 , the wear amount of each of the first grindstone  8  and the second grindstone  10  in the second grinding step S 30  was 1.5. 
     Comparative Experiment 
     In contrast to this result, with use of a grinding wheel in which the first grindstones  8  and the second grindstones  10  are alternately disposed in an annular manner at a predetermined interval of 3.0 mm or more but 4.0 mm or less along the circumferential direction of the base  4 , a comparative experiment in which one surface of the workpiece is ground has been conducted. In the comparative experiment, a first step in which the grinding wheel is rotated in a predetermined direction to grind the workpiece and a second step in which the grinding wheel is rotated in a direction opposite to the predetermined direction to grind the workpiece were substantially the same in wear amount of each of the first grindstones  8  and the second grindstones  10 . Hence, in order to change the grinding characteristic according to a rotational direction, it can be said that an interval between the first grindstone  8  and the second grindstone  10  constituting one grindstone set  6  in the circumferential direction of the base  4  is required to be made smaller than an interval between adjacent ones of the grindstone sets  6  in the circumferential direction of the base  4 . 
     Next, the second embodiment will be described. In the second embodiment, with use of the grinding apparatus  12  mounted thereon with the grinding wheel  2  described above, a workpiece  21  (see  FIG.  8   ) is ground according to the flow chart of the grinding method indicated in  FIG.  4   . As illustrated in  FIG.  8   , the workpiece  21  has a single crystal substrate (the wafer)  23  mainly formed of silicon. The substrate  23  has a disc shape and has a film  25  formed on a front surface  23   a  of the substrate  23 , the film  25  being formed of such a material different from the substrate  23  as silicon oxide (for example, SiO 2 ) film, silicon nitride (for example, Si 3 N 4 ) film, or a metal film. When the workpiece  21  is ground, first, the back surface  23   b  of the substrate  23  positioned on an opposite side of the front surface  23   a  thereof is held under suction on the holding surface  14   a  of the chuck table  14  (holding step S 10 ). 
       FIG.  8    is a diagram illustrating the holding step S 10  according to the grinding method of the second embodiment of the present invention. Subsequent to the holding step S 10 , the first grinding step S 20  is carried out.  FIG.  9    is a diagram illustrating the first grinding step S 20  of the grinding method according to the second embodiment of the present invention. In the first grinding step S 20 , the chuck table  14  is rotated in a predetermined direction at a predetermined value of 100 rpm or higher but 300 rpm or lower, and the spindle  26  is rotated in a predetermined direction at a predetermined value of 3000 rpm or higher but 4000 rpm or lower. Along with these rotational operations, the grinding unit  24  is fed for grinding at a processing feeding speed of a predetermined value of 0.1 µm/s or higher but 2.0 um/s or lower, supplying the grinding water to the processing point. In the first grinding step S 20 , in each of the grindstone sets  6 , in such a manner that the first grindstone  8  is set in front of the second grindstone  10 , the spindle  26  and the grinding wheel  2  are rotated in a predetermined direction. 
     In the first grinding step S 20 , in order to grind a metal film that is likely to generate loading of the grindstones at a time of grinding due to its relative softness, and a silicon oxide film or a silicon nitride film that is likely generate glazing and shedding of the grindstone at a time of grinding due to its relative hardness, the film  25  is ground by effective use of the first grindstone  8  having a high self-sharpening capability due to its relative softness. Accordingly, it is possible to perform grinding on which the grinding characteristic of the first grindstone  8  is reflected. Specifically, it is possible to perform grinding on which the high self-sharpening capability of the first grindstone  8  is reflected. 
     In addition, in the first grinding step S 20 , it is also possible to grind the film  25  by use of the first grindstone  8  having the high self-sharpening capability and by use of the second grindstone  10  the self-sharpening of which is promoted by the abrasive grains  8   b  falling off. Note that, in the first grinding step S 20 , the wear amounts of the first grindstone  8  and the second grindstone  10  are substantially the same. In the first grinding step S 20 , the front surface  23   a  side is ground until the film  25  is removed, and the substrate  23  is exposed. Subsequent to the first grinding step S 20 , the second grinding step S 30  is carried out. 
     In the second grinding step S 30 , first, the grinding unit  24  is raised to such an extent that the first grindstone  8  and the second grindstone  10  are not in contact with the workpiece  11 . The rotational direction and the rotational speed of the chuck table  14  are kept at the same as those in the first grinding step S 20 . Then, in such a manner that the second grindstone  10  is set in front of the first grindstone  8  in each of the grindstone sets  6 , the spindle  26  and the grinding wheel  2  are rotated at a predetermined value of 3000 rpm or higher but 4000 rpm or lower in a direction opposite to the predetermined direction in the first grinding step S 20 . 
     Thereafter, while the spindle  26  is rotated, the grinding unit  24  is fed for grinding at a processing speed of a predetermined value of 0.5 µm/s or higher but 2.0 um/s or lower, supplying the grinding water to the processing point.  FIG.  10    is a diagram illustrating the second grinding step S 30  of the grinding method according to the second embodiment. In the second grinding step S 30 , processing feeding is carried out for a predetermined period of time, and the substrate  23  is ground to a predetermined finished thickness. 
     In the second grinding step S 30 , the second grindstone  10  in each of the grindstone sets  6  grinds mainly the front surface  23   a . Hence, in the second grinding step S 30 , it is possible to perform grinding on which the grinding characteristic of the second grindstone  10  is reflected. Specifically, it is possible to grind the front surface  23   a , reducing the wear amount of the grindstones. Note that, in the second grinding step S 30 , the first grindstone  8  traces the front surface  23   a  having been ground by the second grindstone  10 . As a consequence, the wear amount of the first grindstone  8  is substantially the same as that of the second grindstone  10 . 
     In the second embodiment, with use of the grinding wheel  2 , changing the rotational direction of the spindle  26  allows for exercise of a high self-sharpening capability in the first grinding step S 20  and makes it possible to reduce the wear amount of the grindstone in the second grinding step S 30 . In this manner, with use of one grinding wheel  2  mounted on one spindle  26 , grinding similar to the grinding performed by use of two grinding wheels which have grinding characteristics different from each other can be performed on the workpiece  21 . As a consequence, it is possible to prevent addition of the transferring step between the grinding apparatuses and an increase in footprint for the grinding apparatuses. 
     Next, with reference to  FIG.  11 A  and  FIG.  11 B , a description regarding a modification of the first and second embodiments will be given. In the first and second embodiments described above, the rotational direction of the chuck table  14  is maintained and not changed. However, in the modification, a rotational direction of the chuck table  14  and inclination of the table base  22  are changed according to a rotational direction of the spindle  26  such that a manner of grinding is out-to-in grinding and inner edge grinding. The term, out-to-in grinding, refers to a manner of grinding in which the grindstones grind the workpiece from a peripheral portion of the workpiece to a center thereof. Note that the rotational direction of the chuck table  14  can be changed by the above driving mechanism, and the inclination of the table base  22  can be adjusted by the above inclination adjusting mechanism. 
       FIG.  11 A  is a diagram illustrating a correspondence relation between the rotational direction of the chuck table  14  and a processing region (partial region  14   b ) of the chuck table  14  being substantially parallel to the grinding surface, in a case in which the second grindstone  10  in each of the grindstone sets  6  is set in front of the first grindstone  8 . However, in  FIG.  11 A , the workpiece on the holding surface  14   a  is not illustrated. In a top view of  FIG.  11 A , the rotational directions of the chuck table  14  and the grinding wheel  2  are substantially the same clockwise direction, and the partial region  14   b  is present in the right half of the holding surface  14   a . 
       FIG.  11 B  is a diagram illustrating a correspondence relation between the rotational direction of the chuck table  14  and a processing region (partial region  14   b ) of the chuck table  14  being substantially parallel to the grinding surface, in a case in which the first grindstone  8  in each of the grindstone sets  6  is set in front of the second grindstone  10 . However, in  FIG.  11 B , the workpiece on the holding surface  14   a  is not illustrated. In a top view of  FIG.  11 B , the rotational directions of the chuck table  14  and the grinding wheel  2  are substantially the same counterclockwise direction. In addition, the inclination of the table base  22  is different from the inclination thereof in  FIG.  11 A , and the partial region  14   b  is present in the left half of the holding surface  14   a . 
     In the modification, according to the rotational direction of the spindle  26 , the rotational direction of the chuck table  14  and the inclination of the table base  22  are changed, so that the manner of grinding can be maintained to be out-to-in grinding and inner edge grinding. Besides, a structure, a method, and the like according to the above embodiment may be appropriately modified, and various modifications can be implemented without departing from the scope of the object of the present invention. 
     The present invention is not limited to the details of the above described preferred embodiments. 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.