Patent Publication Number: US-2016236325-A1

Title: Abrasive grindstone

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an abrasive grindstone for grinding a workpiece. 
     2. Description of the Related Art 
     An abrasive grindstone containing a boron compound is used to grind a workpiece formed of a hard brittle material (see Japanese Patent Laid-Open No. 2012-56013, for example). The boron compound has a solid lubricating property and it is therefore considered that the boron compound functions to suppress the consumption of the abrasive grindstone due to grinding of the workpiece. 
     SUMMARY OF THE INVENTION 
     When a grinding load on the abrasive grindstone is high in grinding a workpiece formed of any material inclusive of a hard brittle material, the consumption of the abrasive grindstone is also high in general, so that the frequency of replacement of the abrasive grindstone is increased. Further, heat generated by grinding is not radiated from the abrasive grindstone, but accumulated therein, so that a grinding speed cannot be increased. 
     This problem becomes more remarkable in the case of grinding a workpiece formed of a material having low heat conductivity, such as glass. 
     It is therefore an object of the present invention to provide an abrasive grindstone which can realize a reduction in grinding load, an improvement in heat radiation, or a long life. 
     In accordance with an aspect of the present invention, there is provided an abrasive grindstone for grinding a workpiece, including diamond abrasive grains and a boron compound; the diamond abrasive grains and the boron compound being compounded at a predetermined volume ratio; an average grain size Y of the diamond abrasive grains being set to 0 μm&lt;Y≦50 μm; an average grain size ratio Z of the boron compound to the diamond abrasive grains being set to 0.8≦Z≦3.0. 
     Preferably, the workpiece is a silicon wafer, and the average grain size ratio Z is set to 0.8≦Z≦2.0. Preferably, the predetermined volume ratio between the diamond abrasive grains and the boron compound is set to 1:1 to 1:3. Preferably, the boron compound is selected from group consisting of boron carbide, cubic boron nitride (CBN), and hexagonal boron nitride (HBN). 
     According to the present invention, it is possible to realize a reduction in grinding load on the abrasive grindstone, an improvement in heat radiation, or a long life, so that the productivity can be improved. 
     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 perspective view showing the configuration of a grinding apparatus including an abrasive grindstone according to a preferred embodiment of the present invention; 
         FIG. 2  is a graph showing the results of grinding of an Si wafer by the abrasive grindstone according to the preferred embodiment; 
         FIG. 3  is a graph showing the results of grinding of an Si wafer by the abrasive grindstone according to the preferred embodiment; 
         FIG. 4  is a graph showing the results of grinding of an Si wafer by the abrasive grindstone according to the preferred embodiment; and 
         FIG. 5  is a graph similar to  FIG. 2 , showing the results of grinding of a mirror Si wafer by the abrasive grindstone according to the preferred embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will now be described in detail with reference to the drawings. The present invention is not limited to the preferred embodiment. Further, the components used in the preferred embodiment may include those that can be easily assumed by persons skilled in the art or substantially the same elements as those known in the art. Further, the configurations described below may be suitably combined. Further, the configurations may be variously omitted, replaced, or changed without departing from the scope of the present invention. 
       FIG. 1  is a perspective view showing the configuration of a grinding apparatus including an abrasive grindstone according to a preferred embodiment of the present invention. In  FIG. 1 , the X direction shown by an arrow X is the same as the lateral direction of a grinding apparatus  10 , the Y direction shown by an arrow Y is the same as the longitudinal direction of the grinding apparatus  10 , and the Z direction shown by an arrow Z is the same as the vertical direction perpendicular to the XY plane defined by the X direction and the Y direction. 
     As shown in  FIG. 1 , the grinding apparatus  10  includes a first cassette  11  for storing a plurality of wafers W as a workpiece before grinding, a second cassette  12  for storing the wafers W after grinding, handling means  13  serving commonly as means for taking the wafers W out of the first cassette  11  before grinding and means for taking the wafers W into the second cassette  12  after grinding, positioning means  14  for positioning (centering) the wafers W before grinding, first transfer means  15  for transferring the wafers W before grinding, second transfer means  16  for transferring the wafers W after grinding, three chuck tables  17 ,  18 , and  19  for holding the wafers W under suction, a turn table  20  adapted to be rotated for rotatably supporting the chuck tables  17  to  19 , grinding means  30  and  40  as processing means for performing different kinds of grinding to the wafer W held on each of the chuck tables  17  to  19 , first cleaning means  51  for cleaning the wafers W after grinding, and second cleaning means  52  for cleaning the chuck tables  17  to  19  after grinding. 
     In this grinding apparatus  10 , one of the wafers W stored in the first cassette  11  is taken out of the first cassette  11  and then transferred to the positioning means  14  by the handling means  13 . Thereafter, the wafer W is positioned by the positioning means  14  and then transferred to one of the chuck tables  17  to  19 , e.g., the chuck table  17  in a standby position shown in  FIG. 1 , by the first transfer means  15 . The three chuck tables  17  to  19  are arranged at equal intervals in the circumferential direction of the turn table  20 . Each of the chuck tables  17  to  19  is rotatable about its axis and movable along a circle on the XY plane by the rotation of the turn table  20 . Each of the chuck tables  17  to  19  is adapted to be positioned directly below the grinding means (grinding unit)  30  by the counterclockwise rotation of the turn table  20  at a predetermined angle, e.g., 120 degrees from the standby position in the condition where the wafer W is held under suction. 
     The grinding means  30  functions to perform rough grinding of the wafer W held on each of the chuck tables  17  to  19 . The grinding means  30  is mounted on a wall portion  22  formed at the rear end of a base  21  in the Y direction. A pair of guide rails  31  are provided on the wall portion  22  so as to extend in the Z direction, and a support member  33  is slidably mounted on the guide rails  31  so as to be vertically movable by a motor  32 . The grinding means  30  is supported to the support member  33 , so that the grinding means  30  is vertically movable by the vertical movement of the support member  33  in the Z direction. The grinding means  30  includes a spindle  34   a  rotatably supported, a motor  34  for rotating the spindle  34   a , a wheel mount  35  fixed to the lower end of the spindle  34   a , and a grinding wheel  36  mounted on the lower surface of the wheel mount  35  for grinding the back side of each wafer W. The grinding wheel  36  includes a plurality of abrasive grindstones  37  for rough grinding. The abrasive grindstones  37  are fixed to the lower surface of a base constituting the grinding wheel  36  so as to be arranged annularly along the outer circumference of the base. 
     The rough grinding is performed in the following manner. When the spindle  34   a  is rotated by the motor  34 , the grinding wheel  36  is rotated. At this time, the grinding means  30  is lowered in the Z direction by operating the motor  32  to thereby downward feed the grinding wheel  36  until the abrasive grindstones  37  come into contact with the back side of the wafer W held on the chuck table  17 , for example, and positioned directly below the grinding means  30 . As a result, the back side of the wafer W held on the chuck table  17  is ground by the abrasive grindstones  37  of the grinding wheel  36  being rotated. When the rough grinding of the wafer W held on the chuck table  17  is ended, the turn table  20  is rotated by 120 degrees in the counterclockwise direction, so that the wafer W held on the chuck table  17  is moved to the position directly below the grinding means (grinding unit)  40 . That is, the wafer W is positioned directly below the grinding means  40  after rough grinding. 
     The grinding means  40  functions to perform finish grinding of the wafer W held on each of the chuck tables  17  to  19 . The grinding means  40  is also mounted on the wall portion  22 . A pair of guide rails  41  are provided on the wall portion  22  so as to extend in the Z direction, and a support member  43  is slidably mounted on the guide rails  41  so as to be vertically movable by a motor  42 . The grinding means  40  is supported to the support member  43 , so that the grinding means  40  is vertically movable by the vertical movement of the support member  43  in the Z direction. The grinding means  40  includes a spindle  44   a  rotatably supported, a motor  44  for rotating the spindle  44   a , a wheel mount  45  fixed to the lower end of the spindle  44   a , and a grinding wheel  46  mounted on the lower surface of the wheel mount  45  for grinding the back side of each wafer W. The grinding wheel  46  includes a plurality of abrasive grindstones  47  for finish grinding. The abrasive grindstones  47  are fixed to the lower surface of a base constituting the grinding wheel  46  so as to be arranged annularly along the outer circumference of the base. Thusly, the grinding means  40  has the same basic configuration as that of the grinding means  30 , and the abrasive grindstones  47  are only different in kind from the abrasive grindstones  37 . 
     The finish grinding is performed in the following manner. When the spindle  44   a  is rotated by the motor  44 , the grinding wheel  46  is rotated. At this time, the grinding means  40  is lowered in the Z direction by operating the motor  44  to thereby downward feed the grinding wheel  46  until the abrasive grindstones  47  come into contact with the back side of the wheel W held on the chuck table  17  and positioned directly below the grinding means  40 . As a result, the back side of the wafer W held on the chuck table  17  is ground by the abrasive grindstones  47  of the grinding wheel  46  being rotated. When the finish grinding of the wafer W held on the chuck table  17  is ended, the turn table  20  is rotated by 120 degrees in the counterclockwise direction, so that the wafer W held on the chuck table  17  is returned to the standby position (initial position or load/unload position) shown in  FIG. 1 . At this position, the wafer W whose back side has been finish-ground is transferred to the first cleaning means  51  by the second transfer means  16 . At the first cleaning means  51 , grinding dust is removed from the wafer W by cleaning. Thereafter, the wafer W is taken into the second cassette  12  by the handling means  13 . Further, after the wafer W is transferred from the standby position to the first cleaning means  51 , the chuck table  17  in its empty condition is cleaned by the second cleaning means  52 . Although not specifically described above, the rough grinding and finish grinding of the wafer W held on each of the other chuck tables  18  and  19  are also similarly performed according to the rotational position of the turn table  20 . Further, the loading/unloading of the wafer W to/from each of the other chuck tables  18  and  19  is also similarly performed according to the rotational position of the turn table  20 . 
     Each of the abrasive grindstones  37  and  47  contains diamond abrasive grains and a boron compound. Examples of the diamond abrasive grains include natural diamond, synthetic diamond, and metal coated synthetic diamond. Examples of the boron compound include B4C (boron carbide), CBN (cubic boron nitride), and HBN (hexagonal boron nitride). Each of the abrasive grindstones  37  and  47  is obtained by kneading the diamond abrasive grains and the boron compound with a vitrified bond, resin bond, or metal bond, forming the resultant mixture by using a hot press, and then sintering the resultant formed material. Alternatively, each of the abrasive grindstones  37  and  47  may be obtained by electroforming the diamond abrasive grains and the boron compound with a nickel plating on a base. Further, the volume ratio between the diamond abrasive grains and the boron compound is preferably set to 1:1 to 1:3. 
     Letting X [μm] denote the average grain size of the boron compound and Y [μm] denote the average grain size of the diamond abrasive grains, the average grain size ratio Z (=X/Y) of the boron compound to the diamond abrasive grains in each of the abrasive grindstones  37  and  47  is set to 0.8 Z 3.0. If the average grain size ratio Z is less than 0.8, the function or role of the boron compound as a filler making the abrasive grindstones  37  and  47  brittle becomes large. Further, if the average grain size ratio Z is greater than 3.0, the function or role of the diamond abrasive grains as main abrasive grains becomes smaller than the function or role of the filler, so that the diamond abrasive grains hardly contribute to grinding. Further, the average grain size Y of the diamond abrasive grains is set to 0 μm&lt;Y≦50 μm. The reason why the average grain size Y of the diamond abrasive grains is set to 50 μm or less is that the use of diamond abrasive grains having an average grain size of 50 μm or less is suitable for grinding of each wafer W on which electronic devices are formed. 
     In the case that each wafer W as a workpiece in the preferred embodiment is an Si wafer (silicon wafer) containing Si, the average grain size Y of the diamond abrasive grains in each abrasive grindstone  37  for rough grinding is preferably set to 20 μm≦Y≦50 μm because the average grain size for rough grinding is larger than that for finish grinding. Further, the average grain size Y of the diamond abrasive grains in each abrasive grindstone  47  for finish grinding is preferably set to 0.5 μm≦Y≦1 μm because the average grain size for finish grinding is smaller than that for rough grinding. 
     By setting the average grain size ratio Z of the boron compound to the diamond abrasive grains to 0.8≦Z≦3.0 and setting the average grain size Y of the diamond abrasive grains to 0 μm&lt;Y≦50 μm as described above, the solid lubricating property of the boron compound can be effectively developed in grinding each wafer W. Furthermore, the grinding load on the abrasive grindstones  37  and  47  can be reduced. Since the grinding load on the abrasive grindstones  37  and  47  is reduced, the consumption of the abrasive grindstones  37  and  47  in grinding each wafer W with the abrasive grindstones  37  and  47  can be reduced to result in a long life. Further, the boron compound has high heat conductivity. In particular, CBN and HBN have high heat conductivity. Accordingly, heat radiation from a working point can be improved in grinding the workpiece with the abrasive grindstones  37  and  47 . Thusly, the degree of consumption of the abrasive grindstones  37  and  47  in the grinding apparatus  10  can be suppressed to thereby reduce the frequency of replacement of the abrasive grindstones  37  and  47 . As a result, the productivity in the grinding apparatus  10  can be improved. 
     A comparison was made between a conventional abrasive grindstone and the abrasive grindstone according to the present invention.  FIGS. 2 to 4  are graphs showing the results of grinding by the abrasive grindstone according to the preferred embodiment. In  FIGS. 2 and 4 , the vertical axis represents amperage [A] of an electric current supplied to the motor for rotating the abrasive grindstone, and the horizontal axis represents grinding time [sec] required for grinding of each wafer W. In  FIG. 3 , the vertical axis represents consumption [μm], and the horizontal axis represents the number of wafers W ground, wherein each dot represents the consumption of the abrasive grindstone at the end of grinding of each wafer W. 
     The conventional abrasive grindstone (which will be hereinafter referred to as “conventional sample”) and the abrasive grindstone according to the present invention (which will be hereinafter referred to as “invention samples 1 to 4”) are both abrasive grindstones for rough grinding. That is, the “invention samples 1 to 4” are examples of each abrasive grindstone  37 . On the other hand, the “conventional sample” is an abrasive grindstone excluding a boron compound and containing only diamond abrasive grains, wherein the average grain size Y of the diamond abrasive grains is 20 μm. Each of the “invention samples 1 to 4” is an abrasive grindstone containing both diamond abrasive grains and CBN as a boron compound, wherein the diamond abrasive grains and the boron compound are kneaded together with a vitrified bond and then sintered. The “invention sample 1” is defined so that the average grain size X of the boron compound is 20 μm, the average grain size Y of the diamond abrasive grains is 20 μm, the average grain size ratio Z is 1, and the volume ratio between the boron compound and the diamond abrasive grains is 1. The “invention sample 2” is defined so that the average grain size X of the boron compound is 30 μm, the average grain size Y of the diamond abrasive grains is 20 μm, the average grain size ratio Z is 1.5, and the volume ratio between the boron compound and the diamond abrasive grains is 1. The “invention sample 3” is defined so that the average grain size X of the boron compound is 45 μm, the average grain size Y of the diamond abrasive grains is 20 μm, the average grain size ratio Z is 2.25, and the volume ratio between the boron compound and the diamond abrasive grains is 1. The “invention sample 4” is defined so that the average grain size X of the boron compound is 50 μm, the average grain size Y of the diamond abrasive grains is 20 μm, the average grain size ratio Z is 2.5, and the volume ratio between the boron compound and the diamond abrasive grains is 1. Each wafer W as a workpiece to be ground by the “conventional sample” and the “invention samples 1 to 4” is an Si wafer having an oxide film (SiO 2  film having a thickness of about 600 nm) present on the work surface. That is, a plurality of such wafers W were ground by the “conventional sample” and the “invention samples 1 to 4.” 
       FIG. 2  shows the results of grinding of the wafers W by the “conventional sample” and the “invention sample 1.” In  FIG. 2 , QS 1  and PS 1  denote the results of grinding of the first wafer W by the “conventional sample” and the “invention sample 1,” respectively; QS 2  and PS 2  denote the results of grinding of the second wafer W by the “conventional sample” and the “invention sample 1,” respectively; and QS 3  and PS 3  denote the results of grinding of the third wafer W by the “conventional sample” and the “invention sample 1,” respectively. As apparent from  FIG. 2 , the results of grinding by the “conventional sample” (QS 1  to QS 3 ) are such that no remarkable peaks are present at the start of grinding and the amperage is uniform over the grinding time regardless of the number of wafers W to be ground. On the other hand, the results of grinding by the “invention sample 1” (PS 1  to PS 3 ) are such that peaks higher than the amperage in the case of the “conventional sample” appear at the start of grinding, but the amperage after the occurrence of the peaks is remarkably lower than the amperage in the case of the “conventional sample” regardless of the number of wafers W to be ground. In the case of the “invention sample 1” (also similarly in the case of the “invention samples 2 to 4”), the native oxide (SiO 2 ) formed on the work surface of each wafer W is ground at the start of grinding, so that the amperage showing a grinding load at the start of grinding is higher than that in the case of the “conventional sample” and appears as the peaks. However, after removing the native oxide by grinding, the amperage is rapidly decreased. In other words, the grinding load is greatly reduced as a whole. 
     As described above, the amperage in grinding each wafer W by the “invention sample 1” is lower than that by the “conventional sample” as shown in  FIG. 2 . Accordingly, the consumption of the “invention sample 1” in grinding each wafer W is remarkably reduced as shown in  FIG. 3 . As a result, in the case of grinding the plural wafers W, the gradient of the consumption of the “invention sample 1” (the slope of a line PS shown in  FIG. 3 ) is remarkably smaller than the gradient of the consumption of the “conventional sample” (the slope of a line QS shown in  FIG. 3 ). That is, since the grinding load on the “invention sample 1” is lower than that on the “conventional sample,” the consumption of the “invention sample 1” is lower than that of the “conventional sample,” so that the life of the “invention sample 1” is longer than that of the “conventional sample.” 
       FIG. 4  shows the results of grinding of the n-th wafer W (n is a predetermined number) by the “conventional sample” and the “invention samples 1 to 4.” In  FIG. 4 , QS 4  denotes the result of grinding of the n-th wafer W by the “conventional sample”; PS 4  denotes the result of grinding of the n-th wafer W by the “invention sample 1”; PS 5  denotes the result of grinding of the n-th wafer W by the “invention sample 2”; PS 6  denotes the result of grinding of the n-th wafer W by the “invention sample 3”; and PS 7  denotes the result of grinding of the n-th wafer W by the “invention sample 4.” As apparent from  FIG. 4 , the result of grinding by the “conventional sample” (QS 4 ) is such that no remarkable peak is present at the start of grinding of the oxide film and the amperage is uniform (15 to 16 amperes) over the grinding time. On the other hand, the result of grinding by the “invention sample 1” (PS 4 ) is such that a peak (about 15 amperes) less than the amperage in the case of the “conventional sample” appears at the start of grinding and the amperage after the occurrence of the peak is remarkably lower (12 to 13 amperes) than the amperage in the case of the “conventional sample.” 
     The result of grinding by the “invention sample 2” (PS 5 ) is such that a peak (about 16 amperes) higher than the amperage in the case of the “conventional sample” and the amperage in the case of the “invention sample 1” appears at the start of grinding and the amperage after the occurrence of the peak is remarkably lower (about 12 amperes) than the amperage in the case of the “conventional sample” and substantially the same as the amperage in the case of the “invention sample 1.” The result of grinding by the “invention sample 3” (PS 6 ) is such that a peak (about 18 amperes) higher than the amperage in the case of the “conventional sample” and the amperage in the case of the “invention samples 1 and 2” appears at the start of grinding and the amperage after the occurrence of the peak is remarkably lower (about 12 amperes) than the amperage in the case of the “conventional sample” and substantially the same as the amperage in the case of the “invention samples 1 and 2.” The result of grinding by the “invention sample 4” (PS 7 ) is such that a peak (about 18 amperes) higher than the amperage in the case of the “conventional sample” and the amperage in the case of the “invention samples 1 to 3” appears at the start of grinding and the amperage after the occurrence of the peak is remarkably lower than the amperage in the case of the “conventional sample” and substantially the same as the amperage in the case of the “invention samples 1 to 3.” In the case of the “invention samples 1 to 4,” the amperage increases from 9 amperes to the peak at the start of grinding (in grinding the oxide film) and thereafter decreases to 12 to 13 amperes in grinding the Si wafer. This result shows that the grinding load after removing the oxide film is greatly lower than that in the case of the “conventional sample.” 
     In the case of the “invention samples 1 to 4,” the amperage in grinding each wafer W is lower than that in the case of the “conventional sample.” Accordingly, the grinding load in grinding the plural wafers W is lower than that in the case of the “conventional sample,” so that the consumption of the “invention samples 1 to 4” is lower than that in the case of the “conventional sample.” As a result, the life of the “invention samples 1 to 4” is longer than that of the “conventional sample.” In particular, the “invention samples 1 and 2” are more suitable than the “invention samples 3 and 4” in grinding an Si wafer as the wafer W as the workpiece because the peak at the start of grinding is lower. Accordingly, in the case that the workpiece is an Si wafer, the average grain size ratio Z is preferably set to 0.8≦Z≦2.0. Further, the amperage shown in  FIG. 4  changes according to the grinding apparatus and the peak is preferably lower from the viewpoints of low grinding load and application to the grinding apparatus. That is, the “invention sample 1” or the “invention sample 2” is more preferable than the “invention sample 3” or the “invention sample 4.” 
     While an Si wafer having an oxide film (native oxide) formed on the work surface is used as the workpiece in the preferred embodiment, the wafer W as the workpiece is not limited to an Si wafer, but the wafer W may be an SiC wafer containing SiC, for example. In this case, the average grain size Y of the diamond abrasive grains in each abrasive grindstone  37  for rough grinding is preferably set to 3 μm≦Y≦10 μm because the average grain size for rough grinding is larger than that for finish grinding. Further, the average grain size Y of the diamond abrasive grains in each abrasive grindstone  47  for finish grinding is preferably set to 0.5 μm≦Y≦1 μm because the average grain size for finish grinding is smaller than that for rough grinding. Further, the average grain size ratio Z is preferably set to 1.0≦Z≦2.0. 
     Further, the wafer W as the workpiece may be a mirror Si wafer.  FIG. 5  is a graph showing the results of grinding by the abrasive grindstone according to the present invention in the case that the wafer W is a mirror Si wafer. In  FIG. 5 , the vertical axis represents amperage [A] of an electric current supplied to the motor for rotating the abrasive grindstone, and the horizontal axis represents grinding time [sec] required for grinding of each wafer W.  FIG. 5  shows the results of grinding of a mirror Si wafer by the “conventional sample” and the “invention sample 1.” The mirror Si wafer is an Si wafer having a mirror surface, wherein no oxide film is formed on the mirror surface or a thin oxide film is formed on the mirror surface with the thickness smaller than the thickness of the oxide film formed on the Si wafer shown in  FIGS. 2 to 4 . In  FIG. 5 , QM 1  and PM 1  denote the results of grinding of the first wafer W by the “conventional sample” and the “invention sample 1,” respectively, and QM 2  and PM 2  denote the results of grinding of the second wafer W by the “conventional sample” and the “invention sample 1,” respectively. As apparent from  FIG. 5 , the results of grinding by the “conventional sample” (QM 1  and QM 2 ) are such that no remarkable peaks are present at the start of grinding and the amperage is uniform (about 18 amperes at the maximum) over the grinding time regardless of the number of wafers W to be ground. On the other hand, the results of grinding by the “invention sample 1” (PM 1  and PM 2 ) are such that no peaks appears at the start of grinding and the amperage is lower (about 16 amperes at the maximum) than that in the case of the “conventional sample.” The mirror Si wafer also contains Si as a main component and it is considered that the grinding behavior in grinding the mirror Si wafer is similar to that in grinding the Si wafer shown in  FIGS. 2 to 4  after removing the silicon oxide film. Accordingly, also in grinding the mirror Si wafer by using the “invention samples 2 to 4,” it is possible to exhibit effects similar to those in the case of the Si wafer shown in  FIGS. 2 to 4 . 
     As described above, the amperage in grinding each wafer W by the “invention sample 1” is lower than that by the “conventional sample.” Accordingly, also in the case of grinding the mirror Si wafer, the grinding load on the “invention sample 1” is lower than that on the “conventional sample.” As a result, the consumption of the “invention sample 1” in grinding each wafer W is lower than that of the “conventional sample,” so that the life of the “invention sample 1” is longer than that of the “conventional sample.” Further, since the boron compound has high heat conductivity, heat radiation from the grinding point on the workpiece to be ground by the abrasive grindstones  37  and  47  can be improved. 
     Accordingly, also in the case of grinding the mirror Si wafer, it is possible to suppress a reduction in grinding speed in consideration of heat generated in grinding, so that the productivity can be improved. 
     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.