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 Î¼m<Yâ‰¦50 Î¼m; an average grain size ratio Z of the boron compound to the diamond abrasive grains being set to 0.8â‰¦Zâ‰¦3.0.

Preferably, the workpiece is a silicon wafer, and the average grain size ratio Z is set to 0.8â‰¦Zâ‰¦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 34a rotatably supported, a motor 34 for rotating the spindle 34a, a wheel mount 35 fixed to the lower end of the spindle 34a, 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 34a 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 44a rotatably supported, a motor 44 for rotating the spindle 44a, a wheel mount 45 fixed to the lower end of the spindle 44a, 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 44a 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 [Î¼m] denote the average grain size of the boron compound and Y [Î¼m] 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 Î¼m<Yâ‰¦50 Î¼m. The reason why the average grain size Y of the diamond abrasive grains is set to 50 Î¼m or less is that the use of diamond abrasive grains having an average grain size of 50 Î¼m 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 Î¼mâ‰¦Yâ‰¦50 Î¼m 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 Î¼mâ‰¦Yâ‰¦1 Î¼m 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 0.8â‰¦Zâ‰¦3.0 and setting the average grain size Y of the diamond abrasive grains to 0 Î¼m<Yâ‰¦50 Î¼m 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 [Î¼m], 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 â€œconventional sampleâ€ ) and the abrasive grindstone according to the present invention (which will be hereinafter referred to as â€œinvention samples 1 to 4â€ ) are both abrasive grindstones for rough grinding. That is, the â€œinvention samples 1 to 4â€ are examples of each abrasive grindstone 37. On the other hand, the â€œconventional sampleâ€ 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 Î¼m. Each of the â€œinvention samples 1 to 4â€ 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 â€œinvention sample 1â€ is defined so that the average grain size X of the boron compound is 20 Î¼m, the average grain size Y of the diamond abrasive grains is 20 Î¼m, the average grain size ratio Z is 1, and the volume ratio between the boron compound and the diamond abrasive grains is 1. The â€œinvention sample 2â€ is defined so that the average grain size X of the boron compound is 30 Î¼m, the average grain size Y of the diamond abrasive grains is 20 Î¼m, 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 â€œinvention sample 3â€ is defined so that the average grain size X of the boron compound is 45 Î¼m, the average grain size Y of the diamond abrasive grains is 20 Î¼m, 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 â€œinvention sample 4â€ is defined so that the average grain size X of the boron compound is 50 Î¼m, the average grain size Y of the diamond abrasive grains is 20 Î¼m, 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 â€œconventional sampleâ€ and the â€œinvention samples 1 to 4â€ is an Si wafer having an oxide film (SiO₂film having a thickness of about 600 nm) present on the work surface. That is, a plurality of such wafers W were ground by the â€œconventional sampleâ€ and the â€œinvention samples 1 to 4.â€

FIG. 2 shows the results of grinding of the wafers W by the â€œconventional sampleâ€ and the â€œinvention sample 1.â€ In FIG. 2, QS1 and PS1 denote the results of grinding of the first wafer W by the â€œconventional sampleâ€ and the â€œinvention sample 1,â€ respectively; QS2 and PS2 denote the results of grinding of the second wafer W by the â€œconventional sampleâ€ and the â€œinvention sample 1,â€ respectively; and QS3 and PS3 denote the results of grinding of the third wafer W by the â€œconventional sampleâ€ and the â€œinvention sample 1,â€ respectively. As apparent from FIG. 2, the results of grinding by the â€œconventional sampleâ€ (QS1 to QS3) 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 â€œinvention sample 1â€ (PS1 to PS3) are such that peaks higher than the amperage in the case of the â€œconventional sampleâ€ 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 â€œconventional sampleâ€ regardless of the number of wafers W to be ground. In the case of the â€œinvention sample 1â€ (also similarly in the case of the â€œinvention samples 2 to 4â€ ), the native oxide (SiO₂) 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 â€œconventional sampleâ€ 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 â€œinvention sample 1â€ is lower than that by the â€œconventional sampleâ€ as shown in FIG. 2. Accordingly, the consumption of the â€œinvention sample 1â€ 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 â€œinvention sample 1â€ (the slope of a line PS shown in FIG. 3) is remarkably smaller than the gradient of the consumption of the â€œconventional sampleâ€ (the slope of a line QS shown in FIG. 3). That is, since the grinding load on the â€œinvention sample 1â€ is lower than that on the â€œconventional sample,â€ the consumption of the â€œinvention sample 1â€ is lower than that of the â€œconventional sample,â€ so that the life of the â€œinvention sample 1â€ is longer than that of the â€œconventional sample.â€

FIG. 4 shows the results of grinding of the n-th wafer W (n is a predetermined number) by the â€œconventional sampleâ€ and the â€œinvention samples 1 to 4.â€ In FIG. 4, QS4 denotes the result of grinding of the n-th wafer W by the â€œconventional sampleâ€ ; PS4 denotes the result of grinding of the n-th wafer W by the â€œinvention sample 1â€ ; PS5 denotes the result of grinding of the n-th wafer W by the â€œinvention sample 2â€ ; PS6 denotes the result of grinding of the n-th wafer W by the â€œinvention sample 3â€ ; and PS7 denotes the result of grinding of the n-th wafer W by the â€œinvention sample 4.â€ As apparent from FIG. 4, the result of grinding by the â€œconventional sampleâ€ (QS4) 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 â€œinvention sample 1â€ (PS4) is such that a peak (about 15 amperes) less than the amperage in the case of the â€œconventional sampleâ€ 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 â€œconventional sample.â€

The result of grinding by the â€œinvention sample 2â€ (PS5) is such that a peak (about 16 amperes) higher than the amperage in the case of the â€œconventional sampleâ€ and the amperage in the case of the â€œinvention sample 1â€ 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 â€œconventional sampleâ€ and substantially the same as the amperage in the case of the â€œinvention sample 1.â€ The result of grinding by the â€œinvention sample 3â€ (PS6) is such that a peak (about 18 amperes) higher than the amperage in the case of the â€œconventional sampleâ€ and the amperage in the case of the â€œinvention samples 1 and 2â€ 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 â€œconventional sampleâ€ and substantially the same as the amperage in the case of the â€œinvention samples 1 and 2.â€ The result of grinding by the â€œinvention sample 4â€ (PS7) is such that a peak (about 18 amperes) higher than the amperage in the case of the â€œconventional sampleâ€ and the amperage in the case of the â€œinvention samples 1 to 3â€ 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 â€œconventional sampleâ€ and substantially the same as the amperage in the case of the â€œinvention samples 1 to 3.â€ In the case of the â€œinvention samples 1 to 4,â€ 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 â€œconventional sample.â€

In the case of the â€œinvention samples 1 to 4,â€ the amperage in grinding each wafer W is lower than that in the case of the â€œconventional sample.â€ Accordingly, the grinding load in grinding the plural wafers W is lower than that in the case of the â€œconventional sample,â€ so that the consumption of the â€œinvention samples 1 to 4â€ is lower than that in the case of the â€œconventional sample.â€ As a result, the life of the â€œinvention samples 1 to 4â€ is longer than that of the â€œconventional sample.â€ In particular, the â€œinvention samples 1 and 2â€ are more suitable than the â€œinvention samples 3 and 4â€ 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 0.8â‰¦Zâ‰¦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 â€œinvention sample 1â€ or the â€œinvention sample 2â€ is more preferable than the â€œinvention sample 3â€ or the â€œinvention sample 4.â€

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 Î¼mâ‰¦Yâ‰¦10 Î¼m 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 Î¼mâ‰¦Yâ‰¦1 Î¼m 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 1.0â‰¦Zâ‰¦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 â€œconventional sampleâ€ and the â€œinvention sample 1.â€ 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, QM1 and PM1 denote the results of grinding of the first wafer W by the â€œconventional sampleâ€ and the â€œinvention sample 1,â€ respectively, and QM2 and PM2 denote the results of grinding of the second wafer W by the â€œconventional sampleâ€ and the â€œinvention sample 1,â€ respectively. As apparent from FIG. 5, the results of grinding by the â€œconventional sampleâ€ (QM1 and QM2) 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 â€œinvention sample 1â€ (PM1 and PM2) 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 â€œconventional sample.â€ 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 â€œinvention samples 2 to 4,â€ 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 â€œinvention sample 1â€ is lower than that by the â€œconventional sample.â€ Accordingly, also in the case of grinding the mirror Si wafer, the grinding load on the â€œinvention sample 1â€ is lower than that on the â€œconventional sample.â€ As a result, the consumption of the â€œinvention sample 1â€ in grinding each wafer W is lower than that of the â€œconventional sample,â€ so that the life of the â€œinvention sample 1â€ is longer than that of the â€œconventional sample.â€ 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.