Patent Publication Number: US-11383351-B2

Title: Grinding apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a grinding apparatus including a holding table that holds a workpiece, and a grinding unit (grinding means) having a grinding wheel for grinding the workpiece held by the holding table. 
     Description of the Related Art 
     A plate-shaped workpiece such as a semiconductor wafer is ground by a grinding apparatus (see, for example, Japanese Patent Laid-open No. 2001-284303) to be thinned to a predetermined thickness, and is thereafter divided by a cutting apparatus or the like into individual device chips, which are utilized for various electronic apparatuses and the like. 
     SUMMARY OF THE INVENTION 
     In the case where the wafer is formed of a difficultly grindable material such as gallium nitride (GaN), silicon carbide (SiC) or gallium arsenide (GaAs), there is a problem that the wearing amount of a grindstone of a grinding wheel is large and production cost is raised thereby. In addition, in the case of grinding a wafer formed of a metal or a wafer in which metallic electrodes are partly exposed at a surface to be ground of the wafer, there is a problem that ductility of the metal makes it difficult to perform grinding. 
     It is therefore an object of the present invention to restrain excessive wear of a grindstone and to enable smooth grinding, in the case of grinding a wafer formed of a difficultly grindable material or a wafer including a metal. 
     In accordance with an aspect of the present invention, there is provided a grinding apparatus including a holding table that holds a workpiece, and a grinding unit including a spindle and a grinding wheel that is mounted to the spindle and that grinds the workpiece held by the holding table. The grinding wheel has a grindstone formed by binding abrasive grains with a bonding agent. The grinding apparatus further includes: a grinding water supply unit that supplies grinding water to at least the grindstone when the workpiece held by the holding table is ground by the grinding unit; and a light applying unit that is disposed adjacent to the holding table and that applies light to a grinding surface of the grindstone grinding the workpiece held by the holding table. The light applying unit includes a light emission section that emits light, and a diffusion preventive wall that surrounds the light emission section and that prevents diffusion of the light. 
     Preferably, the light applying unit is disposed on a rotational trajectory of the grinding wheel in such a manner as to face the grinding surface of the grindstone, and the diffusion preventive wall is formed with an entrance section through which the grinding wheel enters and an exit section through which the grinding wheel exits. 
     Preferably, the grindstone has the abrasive grains and photocatalyst grains bound by the bonding agent, and the light applying unit applies the light that excites the photocatalyst grains. 
     In addition, the bonding agent is preferably a vitrified bond. 
     In the grinding apparatus according to the present invention, the grinding wheel has the grindstone formed by binding the abrasive grains with the bonding agent, the grinding apparatus further includes: the grinding water supply unit that supplies grinding water to at least the grindstone when the workpiece held by the holding table is ground by the grinding unit; and the light applying unit that is disposed adjacent to the holding table and that applies light to the grinding surface of the grindstone grinding the workpiece held by the holding table, and the light applying unit includes the light emission section that emits light, and the diffusion preventive wall that surrounds the light emission section and that prevents diffusion of the light. Therefore, during grinding, the light produced by the light emission section is prevented from diffusing by the diffusion preventive wall, whereby the efficiency of application of the light to the grinding surface of the grindstone is enhanced, and, with the grindstone cutting into the workpiece being efficiently made hydrophilic or the like, it is possible to enhance the cooling effect of the grinding water, thereby to restrain wearing of the grindstone, and to enhance a swarf discharging property. Further, with the grindstone made hydrophilic or the like, grinding water is effectively supplied into the processing region where the grindstone grinds the workpiece, and, therefore, processing quality can be prevented from being lowered due to processing heat, and smooth grinding can be achieved even in the case where the workpiece is a wafer formed of a difficultly grindable material. 
     In addition, where the grindstone is formed by binding the abrasive grains and the photocatalyst grains with the bonding agent and the light applying unit applies light that excites the photocatalyst grains, it is ensured that the grinding water supplied exhibits an oxidizing power due to hydroxyl radicals. Therefore, even if the workpiece is a wafer formed, for example, of a difficultly grindable material, it is possible to oxidize the surface to be ground of the workpiece by the strong oxidizing power of the thus produced hydroxyl radicals, to perform grinding while embrittling the surface through the oxidation, and to smoothly grind the workpiece. Similarly, even when the workpiece is a wafer formed of a metal or a wafer in which metallic electrodes are partly exposed at the surface to be ground of the wafer, grinding can be performed while embrittling the metal through oxidation by the strong oxidizing power of hydroxyl radicals, and, therefore, the workpiece can be ground smoothly. 
     With the vitrified bond used as the bonding agent in the grindstone, the property of making hydrophilic or the like the grindstone by application of light thereto can be enhanced more. 
     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 depicting an example of a grinding apparatus; 
         FIG. 2  is a perspective view depicting an example of a grinding wheel; 
         FIG. 3  is a front view depicting, in an enlarged form, part of a grindstone; 
         FIG. 4  is a perspective view depicting an example of positional relation of a grinding unit, a holding table and a light applying unit; 
         FIG. 5  is an end view depicting a state in which a workpiece held by a holding table is being ground by the grindstone; 
         FIG. 6A  is an illustration, as viewed from above, of positional relation of a rotational trajectory of a grinding wheel, a processing region of processing of the workpiece by the grindstone, and the light applying unit during grinding; 
         FIG. 6B  is an illustration, as viewed from a lateral side, of a state in which the grindstone immediately after application of light to a grinding surface is cutting into the workpiece; and 
         FIG. 7  is an end view partly depicting a state in which cleaning water is supplied toward a cover over a light emission section during grinding. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A grinding apparatus  1  depicted in  FIG. 1  is an apparatus that grinds a workpiece W held on a holding table  30  by a grinding unit (grinding means)  7  provided with a grinding wheel  74 . A front side (−Y direction side) on a base  10  of the grinding apparatus  1  is a mounting/detaching region A in which the workpiece W is mounted onto and detached from the holding table  30 , and a rear side on the base  10  is a grinding region B in which grinding of the workpiece W is conducted by the grinding unit  7 . Input means  12  through which an operator inputs processing conditions and the like to the grinding apparatus  1  is disposed on the front side on the base  10 . 
     The holding table  30  is, for example, circular in outer shape, and includes a suction holding section  300  that holds the workpiece W by suction, and a frame body  301  that supports the suction holding section  300 . The suction holding section  300  communicates with a suction source (not depicted), and the workpiece W is suction held on a holding surface  300   a  which is an exposed surface of the suction holding section  300 . The holding surface  300   a  of the holding table  30  is formed in the shape of a conical surface having an extremely gentle inclination with a rotational center of the holding table  30  as a peak. The holding table  30  is surrounded by a cover  31  from the surroundings, is rotatable about an axis set in the vertical direction, and can be reciprocated in a Y-axis direction between the mounting/detaching region A and the grinding region B by Y-axis direction feeding means (not depicted) disposed on the lower side of the cover  31  and a bellows cover  31   a  connected to the cover  31 . 
     A column  11  is erectly provided in the grinding region B, and grinding feeding means  5  for putting the grinding unit  7  into grinding feeding in a Z-axis direction is disposed on a lateral side of the column  11 . The grinding feeding means  5  includes a ball screw  50  having an axis in the Z-axis direction, a pair of guide rails  51  disposed in parallel to the ball screw  50 , a motor  52  that is connected to an upper end of the ball screw  50  and rotates the ball screw  50 , a lift plate  53  a nut in the inside of which is in screw engagement with the ball screw  50  and side portions of which make sliding contact with the guide rails  51 , and a holder  54  that is connected to the lift plate  53  and holds the grinding unit  7 . When the motor  52  rotates the ball screw  50 , the lift plate  53  is reciprocated in the Z-axis direction while guided by the guide rails  51  attendant on this, and the grinding unit  7  held by the holder  54  is put into grinding feeding in the Z-axis direction. 
     The grinding unit  7  includes a spindle  70  having an axial direction in the Z-axis direction, a housing  71  that supports the spindle  70  in a rotatable manner, a motor  72  that rotationally drives the spindle  70 , a mount  73  connected to a tip of the spindle  70 , and a grinding wheel  74  detachably mounted to a lower surface of the mount  73 . 
     The grinding wheel  74  depicted in  FIG. 2  includes an annular wheel base  74   b , and a plurality of substantially rectangular parallelepiped grindstones  74   a  arranged in an annular pattern on a bottom surface (free end portion) of the wheel base  74   b . An upper surface of the wheel base  74   b  is provided with tapped holes  74   c , and jet ports  74   d  through which grinding water is jetted toward the grindstones  74   a . As depicted in  FIG. 3 , the grindstone  74   a  in the present embodiment is formed by mixing diamond abrasive grains P 1  with photocatalyst grains P 2  such as titanium oxide (TiO 2 ) grains, and binding the mixture by a vitrified bond B 1 , which is a vitrified or ceramic bonding agent. The grinding wheel  74  is mounted to a lower surface of the mount  73  by putting screws  73   a  depicted in  FIG. 1  into screw engagement with the tapped holes  74   c  in the wheel base  74   b  through holes provided in the mount  73 . 
     The shape of the grindstone  74   a  may be an integral annular shape, and the photocatalyst grains P 2  may be tin oxide grains, zinc oxide grains, cerium oxide grains or the like. Note that the grindstone  74   a  may not contain the photocatalyst grains P 2 , and a bonding agent other than the vitrified bond may be used as the bonding agent therein. 
     A method of manufacturing the grinding wheel  74  is as follows. First, the vitrified bond B 1  is mixed with diamond abrasive grains P 1  of #1000 in grain size, and the resultant mixture is further mixed with the photocatalyst grains P 2 , followed by stirring. As the vitrified bond B 1 , for example, silicon dioxide (SiO 2 ) is used as a main constituent, which may be admixed with a trace amount of additive for controlling the melting point. Next, this mixture is heated at a predetermined temperature, followed further by pressing to mold the mixture into a substantially rectangular parallelepiped shape. Thereafter, sintering is further conducted at a high temperature for several hours, to produce a grindstone  74   a . The content of the photocatalyst grains P 2  in the grindstone  74   a  is 15 wt %, for example. A plurality of the grindstones  74   a  thus produced are arranged in an annular pattern and fixed to a bottom surface of the wheel base  74   b , to manufacture the grinding wheel  74 . Note that the grain size of the diamond abrasive grains P 1  is not limited to the example in the present embodiment, and may be appropriately modified according to the kind and content of the photocatalyst grains P 2  and the like factors. 
     In the inside of the spindle  70  depicted in  FIG. 1 , a channel  70   a  which communicates with a grinding water supply unit (grinding water supply means)  8  for supplying grinding water to the grindstones  74   a  and which serves as a passage of the grinding water is provided penetrating in the axial direction (Z-axis direction) of the spindle  70 , such that the grinding water having passed through the channel  70   a  can pass through the mount  73  and be jetted from the wheel base  74   b  toward the grindstones  74   a.    
     The grinding water supply unit  8  includes a grinding water source  80  reserving water (for example, pure water), a piping  81  connected to the grinding water source  80  and communicating with the channel  70   a , and a control valve  82  disposed at an arbitrary position on the piping  81  for controlling the quantity of the grinding water supplied, and supplies the grinding water to the grindstones  74   a  at least. 
     As illustrated in  FIGS. 1 and 4 , the grinding apparatus  1  includes a light applying unit (light applying means)  9  that is disposed adjacent to the holding table  30  and that applies light to grinding surfaces (lower surfaces) of the grindstones  74   a  grinding the workpiece W held by the holding table  30 . As depicted in  FIG. 4 , the light applying unit  9  includes, for example, a base section  90  having a substantially arcuate outer shape, a plurality of (in the example illustrated, four) light emission sections  91  alignedly disposed on an upper surface of the base section  90 , a cleaning water supply section  92  that supplies cleaning water (for example, pure water) toward the light emission sections  91 , a cover  93  that prevents dirt from adhering to the light emission sections  91 , and a diffusion preventive wall  94  that surrounds the light emission sections  91  and prevents diffusion of the light. 
     For example, the light emission sections  91  embedded in dents formed in the upper surface of the base section  90  are of light emitting diode (LED) illumination that can emit light of a predetermined wavelength, and can be switched between an ON state and an OFF state by a power source (not depicted). Note that in the case where the photocatalyst grains P 2  contained in the grindstones  74   a  are titanium oxide grains as in the present embodiment, the wavelength of the light (ultraviolet (UV) light) produced by the light emission sections  91  is, for example, preferably 201 nm to 400 nm, and more preferably 201 nm to 365 nm. 
     Note that in the case where the grindstone  74   a  does not contain the photocatalyst grains P 2 , the light emission section  91  is preferably a low pressure mercury lamp or a two-wavelength LED that can emit light at two wavelengths, and is preferably able to emit light of a wavelength of 80 nm to 200 nm (for example, a wavelength of 185 nm) and light of a wavelength of 240 nm to 280 nm (for example, a wavelength of 254 nm). It is natural that in the case where the grindstone  74   a  does not contain the photocatalyst grains P 2 , light of a wavelength of 201 nm to 365 nm may be applied thereto. 
     The plate-shaped cover  93  is composed, for example, of a transparent member such as a glass, and is fixed on the upper surface of the base section  90  in such a manner as to cover the light emission sections  91 . For example, the base section  90  is movable in the vertical direction, such that when performing grinding, the height position of an upper surface of the cover  93  can be set to a desired height position taking into account a grinding feeding position of the grindstones  74   a.    
     The cleaning water supply section  92  includes, for example, a cleaning water source (not depicted) in which water is reserved, and a cleaning water nozzle  920  communicating with the cleaning water source. The cleaning water nozzle  920  is, for example, fixed to a side surface of the base section  90  in such a manner as to lie along the base section  90 , and a plurality of narrow width slit-formed jet ports  920   a  for jetting cleaning water toward the cover  93  covering the light emission sections  91  are aligned in the longitudinal direction of the cleaning water nozzle  920 . The jet ports  920   a  has a shape, a size, an angle relative to the light emission section  91 , etc. set in such a manner that the flow of the cleaning water jetted can be straightened on the upper surface of the cover  93 . 
     The diffusion preventive wall  94  surrounding the light emission sections  91  includes, for example, an outer plate  940  and an inner plate  941 . The outer plate  940  and the inner plate  941  are formed by curving a metallic plate of SUS or the like following the outer shape of the base section  90 , and are fixed individually to side surfaces of the base section  90 . The outer plate  940  and the inner plate  941  are formed with drain ports (not depicted), and cleaning water having been jetted from the cleaning nozzle  920  and having cleaned the upper surface of the cover  93  flows down from the upper surface of the cover  93  and is then discharged through the drain ports to the outside of the diffusion preventive wall  94 . The diffusion preventive wall  94  is formed with an entrance section  94   a  through which the grinding wheel  74  enters and an exit section  94   b  through which the grinding wheel  74  exits. The entrance section  94   a  and the exit section  94   b  are formed in such a size that at least the grindstones  74   a  can pass therethrough. In  FIG. 4 , inside surfaces of the outer plate  940  and the inner plate  941  are smooth curved surfaces, but this is not restrictive; the outer plate  940  and the inner plate  941  may be formed by bending at a predetermined angle and their inside surfaces may be in the form of a polyhedron. 
     An operation of the grinding apparatus  1  depicted in  FIG. 1  in the case of grinding the workpiece W by use of the grinding apparatus  1  will now be described below. 
     The circular disk-shaped workpiece W depicted in  FIG. 1  is, for example, a semiconductor wafer formed of SiC which is difficult to grind, a multiplicity of devices are formed in regions partitioned in a grid pattern by streets (division lines) on a front surface Wa of the workpiece W directed to the lower side in  FIG. 1 , and a protective tape T for protecting the front surface Wa is adhered thereto. A back surface Wb of the workpiece W is a surface to be ground by the grinding wheel  74 . Note that the shape and kind of the workpiece W are not particularly limited, but can be appropriately modified in relation to the grinding wheel  74 ; examples of the applicable workpiece W include a wafer formed of GaAs, GaN or the like, a wafer formed of a metal, and a wafer in which metallic electrodes are partly exposed to the back surface side of the wafer. 
     First, in the mounting/detaching region A, the workpiece W is placed on the holding surface  300   a  of the holding table  30 , with its back surface Wb on the upper side. Then, a suction force generated from a suction source (not depicted) is transmitted to the holding surface  300   a , whereby the holding table  30  holds the workpiece W on the holding surface  300   a  by suction. The workpiece W is in the state of being suction held along the holding surface  300   a  which is a gently inclined conical surface. 
     The holding table  30  is moved in a +Y direction to a position under the grinding unit  7  by the Y-axis direction feeding means (not depicted), whereby aligning of the grinding wheel  74  and the workpiece W held by the holding table  30  is performed. The aligning is conducted, for example, in such a manner that the rotational center of the grinding wheel  74  is deviated by a predetermined distance in the +Y direction from the rotational center of the workpiece W, and the rotational trajectory of the grindstones  74   a  passes the rotational center of the workpiece W. In addition, the inclination of the holding table  30  is adjusted such that the holding surface  300   a  which is a gently inclined conical surface becomes parallel to the grinding surfaces which are lower surfaces of the grindstones  74   a , whereby the back surface Wb of the workpiece W is made to be parallel to the grinding surfaces of the grindstones  74   a.    
     After the aligning of the grinding wheel  74  and the workpiece W is performed, the spindle  70  is rotationally driven by the motor  72 , and, attendant on this, the grinding wheel  74  is rotated counterclockwise as viewed from the +Z direction side, as depicted in  FIG. 5 . In addition, the grinding unit  7  is fed in the −Z direction by the grinding feeding means  5 , and the grinding wheel  74  is gradually lowered in the −Z direction, and the grindstones  74   a  come into contact with the back surface Wb of the workpiece W, whereby grinding is performed. Further, during the grinding, the workpiece W is also rotated attendant on the rotation of the holding table  30  in the counterclockwise direction as viewed from the +Z direction side, so that the grindstones  74   a  perform grinding of the whole area of the back surface Wb of the workpiece W. 
     During the grinding, the grinding water supply unit  8  supplies grinding water into the channel  70   a  in the spindle  70 . As depicted in  FIG. 5 , the grinding water supplied into the channel  70   a  passes through channels  73   b  formed inside the mount  73  at regular intervals in the circumferential direction of the mount  73 , and, further, is jetted through the jet ports  74   d  of the wheel base  74   b  toward the grindstones  74   a.    
     Since the workpiece W is suction held on the holding surface  300   a , which is a gently inclined conical surface, of the holding table  30  along the holding surface  300   a , the grindstones  74   a  make contact with, and grind, the workpiece W in a region E (hereinafter referred to as processing region E) in the rotational trajectory e of the grinding wheel  74  indicated by alternate long and two short dash lines in  FIG. 6A . 
     For example, in a state in which the aligning of the grinding wheel  74  and the holding table  30  has been conducted, the light applying unit  9  disposed adjacent to the holding table  30  is disposed on the immediately upstream side of a position at which the grindstones  74   a  enter the processing region E, as depicted in  FIG. 6A . In addition, as depicted in  FIG. 6B , the light applying unit  9  is disposed on the rotational trajectory e of the grinding wheel  74  in such a manner as to face the grinding surfaces (lower surfaces) of the grindstones  74   a . Attendant on the start of grinding, the light emission sections  91  are put into an ON state, and the light emission sections  91  emit light (UV light) of a wavelength of approximately 365 nm, for example. Then, when the grindstones  74   a  of the grinding wheel  74  being rotated enter the inside of the diffusion preventive wall  94  via the entrance section  94   a , the light produced by the light emission sections  91  is transmitted through the cover  93 , to be applied to the grinding surfaces of the grindstones  74   a  in the inside of the diffusion preventive wall  94 . Since the diffusion preventive wall  94  prevents the light produced by the light emission sections  91  from diffusing and the light is collected onto the grinding surfaces of the grindstones  74   a  passed on the upper side in the inside of the diffusion preventive wall  94 , the efficiency of application of the light to the grinding surfaces of the grindstone  74   a  is improved as compared to that in the related art. 
     By the application of the light, the photocatalyst grains P 2  mixedly present in the grindstones  74   a  are excited, that is, electrons in a valence band of the photocatalyst grains P 2  are excited, whereby two kinds of carriers, namely, electrons and holes, are generated. The holes generated in the photocatalyst grains P 2  mixedly present in the grindstones  74   a  oxidize the grinding water coming into contact with the surfaces of the photocatalyst grains P 2 , thereby producing hydroxyl radicals having a high oxidizing power. Therefore, the grinding water having contacted the grinding surfaces of the grindstones  74   a  exiting through the exit section  94   b  is given the oxidizing power due to the hydroxyl radicals on at least the back surface Wb of the workpiece W. 
     Since the workpiece W formed of SiC is embrittled through oxidation by the hydroxyl radicals thus produced, it becomes easy to grind the workpiece W by the grinding wheel  74 . In addition, the time of presence of the hydroxyl radicals thus produced is very short, and, therefore, other oxidation than that of the back surface Wb of the workpiece W by the grinding water is not generated. In addition, the jetted grinding water functions also to cool the part of contact between the grindstones  74   a  and the back surface Wb of the workpiece W and to remove grinding swarf generated at the back surface Wb of the workpiece W. 
     Note that even where, for example, the workpiece W is a wafer formed of a metal or a wafer in which metallic electrodes are partly exposed to the back surface side of the wafer, grinding can be performed while embrittling the metal through oxidation by the strong oxidizing power of the hydroxyl radicals, so that smooth grinding of the workpiece can be achieved. 
     In addition, the grinding surfaces of the grindstones  74   a  formed using the vitrified bond B 1  as a bonding agent are enhanced in hydrophilicity through, for example, formation of highly polar hydrophilic groups by application of light, so that the grinding water becomes less liable to form droplets on the grinding surfaces of the grindstones  74   a , and the grinding water becomes liable to spread in a water film form over the whole grinding surfaces of the grindstones  74   a . Therefore, the grindstones  74   a  thus made hydrophilic enter the processing region E while being accompanied by much grinding water, and grind the back surface Wb of the workpiece W. With more grinding water entering the area of contact between the back surface Wb of the workpiece W and the grinding surfaces of the grindstones  74   a , generation of frictional heat in the area of contact is restrained. Consequently, excessive wear of the grindstones  74   a  can be restrained, and a swarf discharging property can be enhanced. Further, with the grindstones  74   a  made hydrophilic, the grinding water is effectively supplied into the processing region E where the grindstones  74   a  grind the workpiece W, and, accordingly, processing quality can be prevented from being lowered due to processing heat. 
     Note that in the case where the grindstone  74   a  does not contain the photocatalyst grains P 2 , the light emission sections  91  may apply, for example, UV light of a wavelength of 185 nm and UV light of a wavelength of 254 nm toward the grindstones  74   a . With the UV light of the wavelength of 185 nm applied to the grinding surfaces of the grindstones  74   a , oxygen molecules in air present between the lower surfaces of the grindstones  74   a  and the light emission sections  91  absorb the UV light, whereby oxygen atoms in a ground state are produced. The oxygen atoms thus produced combine with oxygen molecules in the surroundings, to produce ozone. Further, ozone thus produced absorbs the UV light of the wavelength of 254 nm, whereby active oxygen in an excited state is produced. Since active oxygen and ozone have a high oxidizing power, they combine with carbon, hydrogen or the like generated at the grinding surfaces of the grindstones  74   a , to produce highly polar hydrophilic groups on the grinding surfaces of the grindstones  74   a , resulting in that the grindstones  74   a  are made to be hydrophilic. 
     In addition, as depicted in  FIG. 7 , during grinding, the cleaning water supply section  92  supplies cleaning water to the cover  93  over the light emission sections  91 . Specifically, the cleaning water jetted from the cleaning water nozzle  920  onto the cover  93 , whose flow is appropriately straightened, remove dirt such as grinding swarf deposited on the cover  93 , whereby the light generated by the light emission sections  91  during grinding can always be appropriately applied to the processing surfaces of the grindstones  74   a . The cleaning water having flowed down from the upper surface of the cover  93  is discharged through the drain ports (not depicted) to the outside of the diffusion preventive wall  94 . 
     According to an example of experimental results obtained with respect to grinding, grinding of a workpiece W formed of SiC by 50 μm by use of a conventional grinding apparatus took 110 seconds, whereas the grinding by use of the grinding apparatus  1  according to the present invention took only 90 seconds, so that a shortening of grinding time could be realized. In addition, in grinding of a Si surface of a workpiece W, 83% of the whole part of the grindstones of a conventional grinding apparatus was worn where the grinding amount was 100, whereas the wear of the grindstones  74   a  of the grinding apparatus  1  according to the present invention was only 57% of the whole part where the grinding amount was 100. Further, in grinding of a C surface of a workpiece W, 60% of the whole part of the grindstones of a conventional grinding apparatus was worn where the grinding amount was 100, whereas the wear of the grindstones  74   a  of the grinding apparatus  1  according to the present invention was only 39% of the whole part where the grinding amount was 100. 
     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.