Patent Publication Number: US-2023158628-A1

Title: Creep feed grinding apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a creep feed grinding apparatus for grinding a workpiece with a grinding wheel while a grinding unit having a spindle with the grinding wheel mounted on a lower end thereof and a chuck table holding the workpiece under suction thereon are being moved relative to each other in a direction perpendicular to the longitudinal axis of the spindle. 
     Description of the Related Art 
     Pieces of electronic equipment such as cellular phones and personal computers typically incorporate device chips having such devices as integrated circuits (ICs). Device chips are manufactured as follows: First, a plurality of projected dicing lines or streets are established in a grid pattern on the face side of a wafer made of a semiconductor such as silicon, and devices are formed in respective rectangular areas demarcated on the face side of the wafer by the projected dicing lines. Then, a cutting apparatus is used to cut the wafer along the streets into individual pieces as device chips. In recent years, it has been customary to grind the reverse side of a wafer after devices have been formed on the face side thereof, thereby reducing the finished thickness of device chips to be produced from the wafer, with a view to reducing the size and weight of the device chips. 
     Wafers are ground using a creep feed grinding apparatus, for example (see Japanese Patent Laid-open No. 2010-103192). The creep feed grinding apparatus includes a chuck table having a holding surface for holding a workpiece, i.e., a wafer, under suction thereon. The creep feed grinding apparatus also includes a grinding unit disposed above the holding surface. The grinding unit has a cylindrical spindle whose longitudinal axis extends substantially perpendicularly to the holding surface. Usually, the longitudinal axis of the spindle lies substantially parallel to a Z-axis of the creep feed grinding apparatus, e.g., a vertical axis. The spindle has a lower end on which there is mounted an annular grinding wheel by a circular plate mount interposed therebetween. 
     The grinding wheel has an annular base whose lower surface supports thereon an annular array of grindstones spaced at substantially equal intervals along circumferential directions of the annular base. When the spindle is rotated about its central axis, i.e., its longitudinal axis, the grinding wheel is also rotated about its central axis, enabling the grindstones to provide an annular grinding surface along an annular track made up of the lower surfaces of the grindstones as they rotate in unison with the grinding wheel. In order for the creep feed grinding apparatus to operate in a creep feed grinding mode, the workpiece has its face side held under suction on the holding surface with its reverse side exposed upwardly, and the grinding unit is adjusted in its vertical position or height such that the annular grinding surface is slightly lower than the exposed reverse side of the workpiece. Then, the chuck table is moved along an X-axis perpendicular to the Z-axis to cause the grindstones to grind the reverse side of the workpiece in the creep feed grinding mode. 
     In the creep feed grinding mode, the load applied along the X-axis to the outer side surfaces of the grindstones tends to be larger than the load applied along the Z-axis to bottom surfaces of the grindstones. By contrast, in an in-feed grinding mode in which the grinding unit is processing-fed downwardly along the Z-axis while the chuck table disposed below the grinding unit is being rotated, the load applied along the Z-axis to the bottom surfaces of the grindstones tends to be larger than the load applied along the X-axis to the outer side surfaces of the grindstones. 
     In the creep feed grinding mode, depending on the loads applied to the grindstones when the workpiece is ground, the bottom surfaces of the grindstones are liable to wear to a smaller extent than the bottom surfaces of the grindstones liable to wear in the in-feed grinding mode. In the creep feed grinding mode, hence, the bottom surfaces of the grindstones are likely to suffer a grindstone condition failure or malfunction such as grindstone loading. In particular, the bottom surfaces of the grindstones are more likely to suffer a grindstone condition failure when the grindstones grind a substrate of resin in the creep feed grinding mode. 
     SUMMARY OF THE INVENTION 
     Such a grindstone condition failure may be eliminated by a dressing step of dressing the grindstones in addition to a grinding step of grinding the workpiece in the creep feed grinding mode. However, the additional dressing step lowers the efficiency of grinding in the creep feed grinding mode. 
     The present invention has been made in view of the above difficulties. It is an object of the present invention to provide a creep feed grinding apparatus that is capable of eliminating a grindstone condition failure at the bottom surfaces of grindstones without lowering the efficiency of grinding. 
     In accordance with an aspect of the present invention, there is provided a creep feed grinding apparatus including a chuck table having a holding surface for holding a workpiece under suction thereon, a grinding unit having a spindle rotatable about a longitudinal axis thereof and a grinding wheel mounted on a lower end of the spindle, the grinding wheel including an annular base and a plurality of grindstones disposed in an annular array on a surface of the annular base, the grindstones following an annular track upon rotation of the spindle, the annular track having an outside diameter larger than the diameter of the chuck table, a moving mechanism for moving the chuck table and the grinding unit relatively to each other along a predetermined direction perpendicular to the longitudinal axis of the spindle, and a bottom surface state adjusting mechanism for adjusting states of bottom surfaces of the grindstones by cleaning or correcting or cleaning and correcting the bottom surfaces that are held in contact with the workpiece on the holding surface when the grinding unit grinds the workpiece in a creep feed grinding mode, the bottom surface state adjusting mechanism being positioned outside of a relative movement area of the chuck table in which the chuck table and the grinding unit are moved relative to each other by the moving mechanism. 
     Preferably, the bottom surface state adjusting mechanism has a first nozzle for ejecting high-pressure water to the bottom surfaces of the grindstones when the grinding unit grinds the workpiece in the creep feed grinding mode. 
     Preferably, the bottom surface state adjusting mechanism has a second nozzle for ejecting high-pressure water including abrasive grains to the bottom surfaces of the grindstones when the grinding unit grinds the workpiece in the creep feed grinding mode. 
     Preferably, the bottom surface state adjusting mechanism has a third nozzle for ejecting a two-fluid mixture of water and air to the bottom surfaces of the grindstones when the grinding unit grinds the workpiece in the creep feed grinding mode. 
     Preferably, the bottom surface state adjusting mechanism has a dresser for contact with the bottom surfaces of the grindstones when the grinding unit grinds the workpiece in the creep feed grinding mode. 
     Preferably, the bottom surface state adjusting mechanism has a brush for contact with the bottom surfaces of the grindstones when the grinding unit grinds the workpiece in the creep feed grinding mode. 
     Preferably, the bottom surface state adjusting mechanism has a laser beam applying unit including a beam condenser for applying a laser beam to the bottom surfaces of the grindstones when the grinding unit grinds the workpiece in the creep feed grinding mode. 
     The creep feed grinding apparatus according to the aspect of the present invention includes the bottom surface state adjusting mechanism. The bottom surface state adjusting mechanism is positioned outside of the relative movement area of the chuck table in which the chuck table and the grinding unit are moved relative to each other by the moving mechanism. The bottom surface state adjusting mechanism adjusts the states of the bottom surfaces of the grindstones by cleaning or correcting or cleaning and correcting the bottom surfaces that are held in contact with the workpiece on the holding surface when the grinding unit grinds the workpiece in the creep feed grinding mode. For example, when the grinding unit grinds the workpiece in the creep feed grinding mode, the bottom surface state adjusting mechanism eliminates a grindstone condition failure or malfunction such as grindstone loading by cleaning and/or correcting the bottom surfaces of the grindstones that are positioned outside of the chuck table. Consequently, a grindstone condition failure of the bottom surfaces of the grindstones can be eliminated without a reduction in the efficiency of grinding in the creep feed grinding mode. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a side elevational view, partly in cross section, of a creep feed grinding apparatus according to a first embodiment of the present invention; 
         FIG.  2    is a plan view illustrating the manner in which the creep feed grinding apparatus operates in a creep feed grinding mode; 
         FIG.  3 A  is a side elevational view, partly in cross section, of a workpiece and components of the creep feed grinding apparatus at the time the creep feed grinding mode is started; 
         FIG.  3 B  is a side elevational view, partly in cross section, of the workpiece and the components of the creep feed grinding apparatus obtained after a single pass of a chuck table in the creep feed grinding mode; 
         FIG.  4    is a perspective view of the workpiece and the components of the creep feed grinding apparatus that is operating in the creep feed grinding mode; 
         FIG.  5    is a plan view of a creep feed grinding apparatus according to a first modification of the first embodiment; 
         FIG.  6    is a perspective view of the creep feed grinding apparatus according to the first modification of the first embodiment; 
         FIG.  7    is a plan view of a creep feed grinding apparatus according to a second modification of the first embodiment; 
         FIG.  8    is a side elevational view, partly in cross section, of a creep feed grinding apparatus according to a second embodiment of the present invention; 
         FIG.  9    is a side elevational view, partly in cross section, of a creep feed grinding apparatus according to a third embodiment of the present invention; 
         FIG.  10    is a side elevational view, partly in cross section, of a creep feed grinding apparatus according to a fourth embodiment of the present invention; 
         FIG.  11    is a side elevational view, partly in cross section, of a creep feed grinding apparatus according to a fifth embodiment of the present invention; and 
         FIG.  12    is a side elevational view, partly in cross section, of a creep feed grinding apparatus according to a sixth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Identical or similar components are denoted by identical or similar reference characters throughout views. Throughout the drawings, creep feed grinding apparatuses according to the preferred embodiments are illustrated in reference to a three-dimensional coordinate system having X-, Y-, and Z-axes indicated respectively by the arrows X, Y, and Z. The X-axis and the Y-axis lie on a horizontal plane, whereas the Z-axis extends vertically perpendicularly to the horizontal plane. X-axis directions, i.e., forward and rearward directions, extend parallel to the X-axis, and Y-axis directions, i.e., leftward and rightward directions, extend parallel to the Y-axis. Z-axis directions, i.e., upward and downward directions, extend parallel to the Z-axis perpendicular to the X-axis and the Y-axis. 
       FIG.  1    illustrates by way of example, in side elevation, partly in cross section, a creep feed grinding apparatus  2  according to a first embodiment of the present invention. As illustrated in  FIG.  1   , the creep feed grinding apparatus  2  includes a base  4  supporting thereon and housing therein various components of the creep feed grinding apparatus  2 . The base  4  has a recess  4   a  defined in the shape of a rectangular parallelepiped in an upper portion thereof and opening upwardly. The recess  4   a  has a longitudinal axis extending along the X-axis. 
     A circular plate chuck table  6  is movably disposed in the recess  4   a . The chuck table  6  has a circular plate frame  8  that is made of ceramic and that has a circular plate cavity  8   a  defined in an upper portion thereof and opening upwardly. A circular plate porous plate  10  made of porous ceramic is fixedly disposed in the cavity  8   a . The frame  8  and the porous plate  10  have respective upper surfaces lying substantially flush with each other and jointly providing a holding surface  6   a  lying substantially parallel to the X-axis and the Y-axis. The frame  8  has a fluid channel  8   b  that is defined therein and that fluidly connects the porous plate  10  to an unillustrated suction source, such as an ejector. When a negative pressure generated by the suction source is transmitted through the fluid channel  8   b  to the porous plate  10 , a workpiece  11  (see  FIG.  2   ) that is placed on the holding surface  6   a  is held under suction on the holding surface  6   a  under the negative pressure applied to the porous plate  10 . The chuck table  6  is supported on a rectangular X-axis movable plate  12 . 
     The X-axis movable plate  12  is slidably supported on a pair of unillustrated guide rails disposed in the recess  4   a  and extending substantially parallel to the X-axis. A nut  14  is fixedly mounted on a lower surface of the X-axis movable plate  12  and operatively threaded over a screw shaft  16  rotatably disposed between the guide rails and extending along the X-axis. The screw shaft  16  has an end connected to a rotary actuator  18  such as an electric motor for rotating the screw shaft  16  about its central axis. When the rotary actuator  18  is energized, it rotates the screw shaft  16  about its central axis, causing the nut  14  to move the chuck table  6  along the X-axis. The X-axis movable plate  12 , the nut  14 , the screw shaft  16 , the rotary actuator  18 , etc., jointly make up an X-axis moving mechanism  20  for moving the chuck table  6  along the X-axis. 
     The creep feed grinding apparatus  2  includes a support structure  22  shaped as a rectangular parallelepiped protruding upwardly from one end of the base  4  beyond the opening of the recess  4   a  rearwardly of the X-axis moving mechanism  20  in one of the X-axis directions. The support structure  22  is integrally combined with the base  4 , and supports a Z-axis moving mechanism  24  on a front surface thereof facing in the other of the X-axis directions. The Z-axis moving mechanism  24  is fixedly mounted on the front surface of the support structure  22  and includes a pair of guide rails  26  extending parallel to each other vertically along the Z-axis. A hollow cylindrical bottomed holder  28  is slidably mounted on the guide rails  26  for sliding movement along the Z-axis and disposed in front of the guide rails  26 . A nut  30  is fixedly mounted on a rear surface of the holder  28 . The nut  30  is operatively threaded over a screw shaft  32  rotatably disposed between the guide rails  26  and extending along the Z-axis. The screw shaft  32  has an upper end connected to a rotary actuator  34  such as an electric motor for rotating the screw shaft  32  about its central axis. When the rotary actuator  34  is energized, it rotates the screw shaft  32  about its central axis, causing the nut  30  to move the holder  28  along the Z-axis. 
     The holder  28  holds a grinding unit  36  having a hollow cylindrical spindle housing  38  disposed in the holder  28 . The spindle housing  38  is supported on a bottom wall of the holder  28 . The grinding unit  36  includes a cylindrical spindle  40  having a portion rotatably housed in the spindle housing  38 . The spindle  40  has a longitudinal axis extending vertically along the Z-axis. The spindle  40  has an upper end. An unillustrated rotary actuator such as an electric motor is provided near the upper end portion of the spindle  40 . 
     The spindle  40  has a lower end portion protruding downwardly from the holder  28  through a through opening defined in the bottom wall of the holder  28 . A circular plate mount  42  is mounted on a lower end of the spindle  40 . An annular grinding wheel  44  is mounted on the lower end of the spindle  40  through the mount  42 . The grinding wheel  44  has an outside diameter substantially equal to the diameter of the mount  42 . The grinding wheel  44  includes an annular base  46  made of a metal material such as aluminum alloy. The annular base  46  has an upper surface secured to a lower surface of the mount  42  by unillustrated fasteners such as screws. The annular base  46  has a lower surface  46   a  on which there is disposed an annular array of grindstones  48  spaced at substantially equal intervals along circumferential directions of the annular base  46 . Each of the grindstones  48  is shaped substantially like a block and is made up of abrasive grains of diamond or cubic boron nitride (cBN) and a binder, i.e., a binding material, of metal, resin, or ceramic holding the abrasive grains together. 
     When the Z-axis moving mechanism  24  is operated, it moves the grinding unit  36  along the Z-axis relative to the chuck table  6 , i.e., toward or away from the chuck table  6 . The grinding unit  36  including the grinding wheel  44 , i.e., the grindstones  48 , is thus adjusted in vertical position or height with respect to the chuck table  6 . When the spindle  40  is rotated about its central axis by the rotary actuator connected thereto, the grindstones  48  provide an annular grinding surface  48   a  (see  FIG.  3 A ) along an annular track followed by bottom surfaces  48   d  of the grindstones  48  as they rotate in unison with the grinding wheel  44 . In  FIG.  3 A , the annular grinding surface  48   a  is illustrated in its vertical position along the Z-axis. 
       FIG.  2    illustrates in plan the manner in which the creep feed grinding apparatus  2  operates in a creep feed grinding mode. The annular track followed by the bottom surfaces  48   d  of the grindstones  48  at the time the spindle  40  is rotated about its central axis, i.e., the annular grinding surface  48   a , has an outside diameter  48   b  that is larger than a diameter  6   b  of the chuck table  6 . For example, the outside diameter  48   b  is 500 mm and the diameter  6   b  is 300 mm. The outside diameter  48   b  may be larger than the diameter  6   b  by 60 mm or more. As viewed in plan on an X-Y plane, the outside diameter  48   b  of the grinding surface  48   a  has a center  48   c  and the diameter  6   b  of the chuck table  6  has a center  6   c , the centers  48   c  and  6   c  being positioned on a straight line along the X-axis. When the chuck table  6  is relatively moved along in one of the X-axis directions to a position directly below the grinding wheel  44 , i.e., the grinding unit  36 , and is vertically aligned with the grinding wheel  44 , the chuck table  6  is positioned radially inwardly of an inner circumferential edge of the grinding wheel  44  as viewed in plan as indicated by the broken lines in  FIG.  2   . 
     The creep feed grinding apparatus  2  according to the first embodiment includes a bottom surface state adjusting unit  50 . The bottom surface state adjusting unit  50  has a first nozzle, i.e., a bottom surface state adjusting mechanism,  52  disposed outside of a relative movement area B (see  FIG.  2   ) in which the chuck table  6  is movable relative to the grinding unit  36  along the Y-axis. The first nozzle  52  is kept in a fixed position relative to the grinding unit  36 . For example, the first nozzle  52  is fixed to the base  4  at a position directly below the grinding surface  48   a . The distance between the first nozzle  52  and the grinding surface  48   a  positioned directly above the first nozzle  52  during a grinding process is adjusted in advance depending on the speed of high-pressure water  54  ejected from the first nozzle  52 . 
     As illustrated in  FIG.  3 A , the first nozzle  52  is fluidly connected to a high-pressure water supply source  56 . The high-pressure water supply source  56  has an unillustrated tank containing pure water therein and an unillustrated pump for increasing the pressure of the pure water supplied from the tank to a predetermined pressure. During the creep feed grinding mode, the first nozzle  52  ejects the high-pressure water  54  that has been pressurized to 0.1 MPa or higher, e.g., a predetermined pressure value ranging from 2 MPa to 13 MPa, upwardly to the grindstones  48 , thereby adjusting the bottom surfaces  48   d  (see  FIG.  3 A ) of the grindstones  48 . 
     When the creep feed grinding apparatus  2  is to perform creep feed grinding on the workpiece  11 , the chuck table  6  holds a face side  11   a  of the workpiece  11  under suction thereon such that a reverse side  11   b  thereof is exposed upwardly. Providing devices are formed on the face side  11   a , a protective tape of resin is affixed to the face side  11   a  to protect the devices, and then the chuck table  6  holds the face side  11   a  under suction thereon. The chuck table  6  holds the face side  11   a  under suction thereon in a loading/unloading area A 1  positioned on a front side of the creep feed grinding apparatus  2 . After the chuck table  6  has held the face side  11   a  under suction thereon, the spindle  40  is rotated about its central axis at a predetermined rotational speed, and the Z-axis moving mechanism  24  adjusts the height or vertical position of the grinding surface  48   a  to a position between the holding surface  6   a  and the reverse side  11   b  of the workpiece  11  (see  FIG.  3 A ) such that the bottom surfaces  48   d  of the grindstones  48  come into contact with the reverse side  11   b . The rotational speed of the spindle  40  may be set to an appropriate value depending on the outside diameter  48   b  of the grinding surface  48   a . For example, if the outside diameter  48   b  is 500 mm, then the rotational speed of the spindle  40  is set to 2000 rpm, and if the outside diameter  48   b  is 300 mm, then the rotational speed of the spindle  40  is set to 3200 rpm. 
     After the Z-axis moving mechanism  24  has adjusted the height or vertical position of the grinding surface  48   a , the first nozzle  52  starts ejecting the high-pressure water  54  upwardly, and the chuck table  6  starts moving toward the grinding unit  36  in a processing feed direction indicated by the arrow in  FIGS.  2  and  3 A , whereupon the creep feed grinding apparatus  2  starts grinding the workpiece  11  in the creep feed grinding mode, or more specifically, the grindstones  48  start grinding the reverse side  11   b  as they move along the annular track in contact therewith.  FIG.  3 A  illustrates in side elevation, partly in cross section, the workpiece  11  and components of the creep feed grinding apparatus  2  at the time the creep feed grinding mode is started. In the creep feed grinding mode, the X-axis moving mechanism  20  moves the chuck table  6  to a predetermined area A 2  on a rear side of the creep feed grinding apparatus  2  at a predetermined speed of 10 mm/s., for example. 
     According to the first embodiment, the predetermined area A 2  is positioned directly below the grinding unit  36 . The chuck table  6  that has been moved to the predetermined area A 2  is positioned radially inwardly of the inner circumferential edge of the grinding surface  48   a  as viewed in plan on the X-Y plane (see  FIG.  2   ). As the grindstones  48  move to the predetermined area A 2 , their side surfaces and the bottom surfaces  48   d  grind the reverse side  11   b  of the workpiece  11 , leaving a plurality of arcuate saw marks  11   c  (see  FIG.  2   ) on the reverse side  11   b  that are successively arranged along the processing feed direction.  FIG.  3 B  illustrates in side elevation, partly in cross section, the workpiece  11  and components of the creep feed grinding apparatus  2  obtained after a single pass of the chuck table  6  in the creep feed grinding mode. 
     A single pass refers to a single operation in which the chuck table  6  and the grinding unit  36  are to be moved relative to each other in a predetermined direction in order to move the chuck table  6  from a position outside of the grinding wheel  44  in the X-Y plane until it is positioned directly below the grinding wheel  44 . According to the first embodiment, a single progression of the chuck table  6  from outside of the grinding wheel  44  to the position directly below the grinding wheel  44  in a direction along the X-axis from the loading/unloading area A 1  (see  FIG.  3 A ) to the predetermined area A 2  (see  FIG.  3 B ) is referred to as a single pass. According to the first embodiment, when the workpiece  11  is ground by the grinding wheel  44  in the single pass in the creep feed grinding mode, the first nozzle  52  ejects the high-pressure water  54  to the bottom surfaces  48   d  of the grindstones  48 , thereby cleaning and/or correcting, i.e., cleaning and correcting or cleaning or correcting, the bottom surfaces  48   d  with the high-pressure water  54 .  FIG.  4    illustrates in perspective the workpiece  11  and the components of the creep feed grinding apparatus  2  that is operating in the creep feed grinding mode. 
     According to the first embodiment, since the high-pressure water  54  is ejected to the bottom surfaces  48   d  of the grindstones  48 , it is possible to at least remove grinding debris or swarf from the grindstones  48 , dress the grindstones  48 , or correct the shape of the grindstones  48  at the bottom surfaces  48   d  thereof, thereby eliminating a grindstone condition failure of the bottom surfaces  48   d  in the creep feed grinding mode. Consequently, a grindstone condition failure of the bottom surfaces  48   d  can be eliminated without a reduction in the efficiency of grinding in the creep feed grinding mode. Furthermore, inasmuch as the first nozzle  52  is positioned outside of the relative movement area B (see  FIG.  2   ) in which the chuck table  6  is movable relative to the grinding unit  36 , the space outside of the relative movement area B is effectively utilized. 
     For grinding and thinning down the workpiece  11  to a desired finished thickness, the creep feed grinding apparatus  2  may perform the creep feed grinding mode by moving the chuck table  6  in two or more passes. Specifically, when the creep feed grinding apparatus  2  is to perform the creep feed grinding mode by moving the chuck table  6  in a second pass, the grinding unit  36  is lifted to a height where the grindstones  48  will not contact the workpiece  11  after the chuck table  6  has been moved to the position directly below the grinding unit  36  in the first pass. Then, the chuck table  6  is moved from the predetermined area A 2  back to the loading/unloading area A 1  where the chuck table  6  does not underlie the grinding wheel  44  as viewed in plan on the X-Y plane. Thereafter, the grinding unit  36  is lowered to a position for contact with the workpiece  11 , and then the chuck table  6  is moved along the X-axis from the loading/unloading area A 1  (see  FIG.  3 A ) to the predetermined area A 2  (see  FIG.  3 B ) to grind the workpiece  11  in the second pass in the creep feed grinding mode. The grinding wheel  44  and the chuck table  6  may be moved in the same fashion for a third pass or third and subsequent passes until the workpiece  11  is thinned down to a desired finished thickness. While the creep feed grinding apparatus  2  is performing the creep feed grinding mode, the first nozzle  52  continuously ejects the high-pressure water  54 . However, the first nozzle  52  stops ejecting the high-pressure water  54  during the movement of the chuck table  6  from the predetermined area A 2  back to the loading/unloading area A 1 . 
     Two or more first nozzles  52  may be disposed directly below the grinding surface  48   a  unless they are in interference with the relative movement area B. For example, two or more first nozzles  52  may be disposed outside of the relative movement area B on one side or respective both sides thereof along the Y-axis. In particular, if two first nozzles  52  are disposed diametrically across the center  48   c  on the outside diameter  48   b  of the grinding surface  48   a , then regardless of the direction in which the spindle  40  is rotated, the bottom surfaces  48   d  of the grindstones  48  can be cleaned and/or corrected by the high-pressure water  54  immediately before or after the bottom surfaces  48   d  contact the workpiece  11 . Accordingly, the degree of freedom of the spindle  40  can be secured. 
     A first modification of the first embodiment will be described below with reference to  FIGS.  5  and  6   .  FIG.  5    illustrates in plan a creep feed grinding apparatus  2   a  according to the first modification, and  FIG.  6    illustrates in perspective the creep feed grinding apparatus  2   a  according to the first modification. According to the first modification, the chuck table  6  is not moved by the X-axis moving mechanism  20  and remains stationary at all times. On the other hand, the support structure  22  to which the Z-axis moving mechanism  24  is fixed is movable along the X-axis by a moving mechanism similar to the X-axis moving mechanism  20 . The moving mechanism has an unillustrated X-axis movable plate that supports the support structure  22  thereon. 
     The first nozzle  52  is fixedly mounted on the holder  28  or the X-axis movable plate, so that the first nozzle  52  is movable with the support structure  22  along the X-axis. Other details of the creep feed grinding apparatus  2   a  according to the first modification are identical to those of the creep feed grinding apparatus  2  according to the first embodiment. According to the first modification, it is also possible to at least remove grinding debris or swarf from the grindstones  48 , dress the grindstones  48 , or correct the shape of the grindstones  48  at the bottom surfaces  48   d  thereof in the creep feed grinding mode. Consequently, a grindstone condition failure of the bottom surfaces  48   d  can be eliminated without a reduction in the efficiency of grinding in the creep feed grinding mode. 
     A second modification of the first embodiment will be described below with reference to  FIG.  7   .  FIG.  7    illustrates in plan a creep feed grinding apparatus  2   b  according to the second modification. According to the second modification, the chuck table  6  also remains stationary at all times, and the support structure  22  to which the Z-axis moving mechanism  24  is fixed is also movable along the X-axis. According to the second modification, however, the first nozzle  52  is fixed in position in the vicinity of the chuck table  6  and is not movable along the X-axis. 
     The second modification is different from the first modification in that the first nozzle  52  is disposed on a straight line parallel to the Y-axis across the center  6   c  (see  FIG.  2   ) as viewed in plan on the X-Y plane, directly below a relative movement area in which the grinding surface  48   a  is movable relative to the chuck table  6  outside of the relative movement area B. Other details of the creep feed grinding apparatus  2   b  according to the second modification are identical to those of the creep feed grinding apparatus  2   a  according to the first modification. According to the second modification, it is also possible to at least remove grinding debris or swarf from the grindstones  48 , dress the grindstones  48 , or correct the shape of the grindstones  48  at the bottom surfaces  48   d  thereof in the creep feed grinding mode. 
     A second embodiment of the present invention will be described below.  FIG.  8    illustrates in side elevation, partly in cross section, a creep feed grinding apparatus  62   a  according to the second embodiment. The creep feed grinding apparatus  62   a  includes a bottom surface state adjusting unit  50   a . The bottom surface state adjusting unit  50   a  has a second nozzle, i.e., a bottom surface state adjusting mechanism,  52   a  whose relative position with respect to the grinding unit  36  is fixed. The second nozzle  52   a  illustrated in  FIG.  8    is fixed to the base  4  and disposed at a position directly below the grinding surface  48   a  outside of the relative movement area B (see  FIG.  2   ) in which the chuck table  6  is movable relative to the grinding unit  36 . 
     The second nozzle  52   a  ejects high-pressure water  54   a   2  that includes abrasive grains  54   a   1  and that is pressurized to 0.1 MPa or higher, e.g., a predetermined pressure value ranging from 2 MPa to 13 MPa, upwardly to the grindstones  48 . The abrasive grains  54   a   1  have an average particle size smaller than the average particle size of the abrasive grains of the grindstones  48 . The second nozzle  52   a  is fluidly connected to an abrasive-grain-containing high-pressure water supply source  56   a . The abrasive-grain-containing high-pressure water supply source  56   a  has an unillustrated tank containing pure water mixed with abrasive grains  54   a   1  and an unillustrated pump for increasing the pressure of the pure water mixed with the abrasive grains  54   a   1  supplied from the tank to a predetermined pressure. 
     According to the second embodiment, when the workpiece  11  is ground by the grinding wheel  44  in the creep feed grinding mode, the second nozzle  52   a  ejects the high-pressure water  54  containing the abrasive grains  54   a   1  to the bottom surfaces  48   d  of the grindstones  48 , thereby cleaning and/or correcting, i.e., cleaning and correcting or cleaning or correcting, the bottom surfaces  48   d  with the high-pressure water  54  containing the abrasive grains  54   a   1 . Thus, it is possible to at least remove grinding debris or swarf from the grindstones  48 , dress the grindstones  48 , or correct the shape of the grindstones  48  at the bottom surfaces  48   d  thereof. Consequently, a grindstone condition failure of the bottom surfaces  48   d  can be eliminated without a reduction in the efficiency of grinding in the creep feed grinding mode. 
     In  FIG.  8   , the single second nozzle  52   a  is disposed directly below the grinding surface  48   a . However, two or more second nozzles  52   a  may be disposed directly below the grinding surface  48   a  unless they are in interference with the relative movement area B. For example, two or more second nozzles  52   a  may be disposed outside of the relative movement area B on one side or respective both sides thereof along the Y-axis. In particular, if two second nozzles  52   a  are disposed diametrically across the center  48   c  on the outside diameter  48   b  of the grinding surface  48   a , then as described above, the degree of freedom of the spindle  40  can be secured. The first modification or the second modification described above is also applicable to the creep feed grinding apparatus  62   a  according to the second embodiment. 
     A third embodiment of the present invention will be described below.  FIG.  9    illustrates in side elevation, partly in cross section, a creep feed grinding apparatus  62   b  according to the third embodiment. The creep feed grinding apparatus  62   b  includes a bottom surface state adjusting unit  50   b . The bottom surface state adjusting unit  50   b  has a third nozzle, i.e., a bottom surface state adjusting mechanism,  52   b  whose relative position with respect to the grinding unit  36  is fixed. The third nozzle  52   b  illustrated in  FIG.  9    is fixed to the base  4  and disposed at a position directly below the grinding surface  48   a  outside of the relative movement area B (see  FIG.  2   ) in which the chuck table  6  is movable relative to the grinding unit  36 . 
     The third nozzle  52   b  ejects a two-fluid mixture  54   b  of pure water  54   b   1  and air  54   b   2  upwardly. For example, pure water  54   b   1  that has been pressurized to 0.8 MPa and air  54   b   2  that has been pressurized to 0.3 MPa are independently supplied to the third nozzle  52   b  in which they are mixed together, and they are injected as the two-fluid mixture  54   b  upwardly from the third nozzle  52   b.    
     The third nozzle  52   b  is fluidly connected to a two-fluid mixture supply source  56   b  through a conduit for pure water  54   b   1  and a conduit for air  54   b   2 . The two-fluid mixture supply source  56   b  includes an unillustrated pure water supply source having an unillustrated pump for supplying pressurized pure water  54   b   1  and an unillustrated tank containing pure water  54   b   1  therein. The two-fluid mixture supply source  56   b  also includes an unillustrated air supply source having an unillustrated pump for supplying pressurized air  54   b   2  and an unillustrated tank containing air  54   b   2  therein. 
     According to the third embodiment, when the workpiece  11  is ground by the grinding wheel  44  in the creep feed grinding mode, the third nozzle  52   b  ejects the two-fluid mixture  54   b  to the bottom surfaces  48   d  of the grindstones  48 , thereby cleaning and/or correcting the bottom surfaces  48   d  with the two-fluid mixture  54   b . Thus, it is possible to at least remove grinding debris or swarf from the grindstones  48 , dress the grindstones  48 , or correct the shape of the grindstones  48  at the bottom surfaces  48   d  thereof. Consequently, a grindstone condition failure of the bottom surfaces  48   d  can be eliminated without a reduction in the efficiency of grinding in the creep feed grinding mode. 
     In  FIG.  9   , the single third nozzle  52   b  is disposed directly below the grinding surface  48   a  outside of the relative movement area B. However, two or more third nozzles  52   b  may be disposed directly below the grinding surface  48   a . For example, two or more third nozzles  52   b  may be disposed outside of the relative movement area B on one side or respective both sides thereof along the Y-axis. In particular, if two third nozzles  52   b  are disposed diametrically across the center  48   c  on the outside diameter  48   b  of the grinding surface  48   a , then as described above, the degree of freedom of the spindle  40  can be secured. The first modification or the second modification described above is also applicable to the creep feed grinding apparatus  62   b  according to the third embodiment. 
     A fourth embodiment of the present invention will be described below.  FIG.  10    illustrates in side elevation, partly in cross section, a creep feed grinding apparatus  62   c  according to the fourth embodiment. The creep feed grinding apparatus  62   c  includes a bottom surface state adjusting unit  50   c . The bottom surface state adjusting unit  50   c  has a circular plate dresser, i.e., a bottom surface state adjusting mechanism,  52   c   1  whose relative position with respect to the grinding unit  36  is fixed. The dresser  52   c   1  is supported on and fixed to a cylindrical base  52   c   2 . The base  52   c   2  is mounted on the base  4  by an unillustrated lifting and lowering mechanism for selectively lifting and lowering the base  52   c   2  along the Z-axis. The dresser  52   c   1  is disposed at a position directly below the grinding surface  48   a  outside of the relative movement area B (see  FIG.  2   ). 
     The dresser  52   c   1  has a diameter ranging from 1 cm to 5 cm and a thickness ranging from 1 mm to 5 mm, for example. The dresser  52   c   1  may be referred to as a dressing board. The diameter of the dresser  52   c   1  is selected depending on the width of each of the grindstones  48 . The dresser  52   c   1  is made up of a binder such as a vitrified bond and abrasive grains of white alundum (WA), green carbon (GC), or the like that are bound together by the binder. 
     When the creep feed grinding apparatus  62   c  starts to operate in the creep feed grinding mode, the lifting and lowering mechanism lifts the dresser  52   c   1  to a lifted position in which the height or vertical position of an upper surface thereof is aligned with the height or vertical position of the grinding surface  48   a . When the creep feed grinding apparatus  62   c  is not grinding the workpiece  11 , e.g., when the creep feed grinding apparatus  62   c  is serviced for maintenance, the lifting and lowering mechanism lowers the dresser  52   c   1  to a predetermined lowered position out of contact with the bottom surfaces  48   d . The lifting and lowering mechanism may have an actuating unit such as an air cylinder for positioning the dresser  52   c   1  selectively in the lifted position and the lowered position and a ball-screw-type moving mechanism for finely adjusting the height of the base  52   c   2  depending on the extent to which the dresser  52   c   1  is worn. 
     According to the fourth embodiment, when the workpiece  11  is ground by the grinding wheel  44  in the creep feed grinding mode, the dresser  52   c   1  is held in contact with the bottom surfaces  48   d  of the grindstones  48 , thereby cleaning and/or correcting the bottom surfaces  48   d . Thus, it is possible to at least remove grinding debris or swarf from the grindstones  48 , dress the grindstones  48 , or correct the shape of the grindstones  48  at the bottom surfaces  48   d  thereof. Consequently, a grindstone condition failure of the bottom surfaces  48   d  can be eliminated without a reduction in the efficiency of grinding in the creep feed grinding mode. In  FIG.  10   , the single dresser  52   c   1  is disposed directly below the grinding surface  48   a  outside of the relative movement area B. However, two or more dressers  52   c   1  may be disposed directly below the grinding surface  48   a.    
     Furthermore, two or more dressers  52   c   1  may be disposed outside of the relative movement area B on one side or respective both sides thereof along the Y-axis. Since the load on each of the two or more dressers  52   c   1  is smaller than the load on the single dresser  52   c   1  when the grindstones  48  are dressed, the two or more dressers  52   c   1  may be replaced less frequently. In particular, if two dressers  52   c   1  are disposed diametrically across the center  48   c  on the outside diameter  48   b  of the grinding surface  48   a , then as described above, the degree of freedom of the spindle  40  can be secured. The first modification or the second modification described above is also applicable to the creep feed grinding apparatus  62   c  according to the fourth embodiment. 
     A fifth embodiment of the present invention will be described below.  FIG.  11    illustrates in side elevation, partly in cross section, a creep feed grinding apparatus  62   d  according to the fifth embodiment. The creep feed grinding apparatus  62   d  includes a bottom surface state adjusting unit  50   d . The bottom surface state adjusting unit  50   d  has a brush, i.e., a bottom surface state adjusting mechanism,  52   d  whose relative position with respect to the grinding unit  36  is fixed. According to the fifth embodiment, the brush  52   d  is a tubular brush having bristles  52   d   1  made of such resin as polyamide or polyester and a tube  52   d   2  bundling up the lower ends of the bristles  52   d   1 . According to the present invention, the brush is not limited to a tubular brush, and may be any of brushes having other shapes. 
     The brush  52   d  illustrated in  FIG.  11    is fixed to the base  4  at a position directly below the grinding surface  48   a  outside of the relative movement area B. The brush  52   d  is connected to an unillustrated lifting and lowering mechanism for selectively lifting and lowering the brush  52   d  along the Z-axis. When the creep feed grinding apparatus  62   d  starts to operate in the creep feed grinding mode, the lifting and lowering mechanism lifts the brush  52   d  to a lifted position in which the height or vertical position of upper ends of the bristles  52   d   1  is aligned with the height or vertical position of the grinding surface  48   a . When the creep feed grinding apparatus  62   d  is not grinding the workpiece  11 , e.g., when the creep feed grinding apparatus  62   d  is serviced for maintenance, the lifting and lowering mechanism lowers the brush  52   d  to a predetermined lowered position out of contact with the bottom surfaces  48   d.    
     According to the fifth embodiment, when the workpiece  11  is ground by the grinding wheel  44  in the creep feed grinding mode, the brush  52   d  is held in contact with the bottom surfaces  48   d  of the grindstones  48 , thereby cleaning and/or correcting the bottom surfaces  48   d . Thus, it is possible to at least remove grinding debris or swarf from the grindstones  48 , dress the grindstones  48 , or correct the shape of the grindstones  48  at the bottom surfaces  48   d  thereof. Consequently, a grindstone condition failure of the bottom surfaces  48   d  can be eliminated without a reduction in the efficiency of grinding in the creep feed grinding mode. 
     In  FIG.  11   , the single brush  52   d  is disposed directly below the grinding surface  48   a  outside of the relative movement area B. However, two or more brushes  52   d  may be disposed directly below the grinding surface  48   a . Further, two or more brushes  52   d  may be disposed outside of the relative movement area B on one side or respective both sides thereof along the Y-axis. In particular, if two brushes  52   d  are disposed diametrically across the center  48   c  on the outside diameter  48   b  of the grinding surface  48   a , then as described above, the degree of freedom of the spindle  40  can be secured. The first modification or the second modification described above is also applicable to the creep feed grinding apparatus  62   d  according to the fifth embodiment. 
     A sixth embodiment of the present invention will be described below.  FIG.  12    illustrates in side elevation, partly in cross section, a creep feed grinding apparatus  62   e  according to the sixth embodiment. The creep feed grinding apparatus  62   e  includes a bottom surface state adjusting unit  50   e . The bottom surface state adjusting unit  50   e  has a laser beam applying unit  52   e . The laser beam applying unit  52   e  has a laser oscillator  52   e   1  for emitting a pulsed laser beam L. The laser oscillator  52   e   1  includes a laser diode for generating and emitting laser radiation and an unillustrated pulse generator for controlling pulse characteristics including a pulse duration, a repetitive frequency, etc. of the pulsed laser beam L. 
     The pulse generator controls the laser emission from the laser diode. The laser emission from the laser diode is amplified by a rare-earth-doped fiber, e.g., an ytterbium (Yb)-doped fiber, enabling the laser oscillator  52   e   1  to emit the pulsed laser beam L that has a predetermined wavelength of 1030 nm, for example. The laser beam L emitted from the laser oscillator  52   e   1  is reflected by a mirror  52   e   2  and travels through by a lens  52   e   4  in a beam condenser, i.e., a bottom surface state adjusting mechanism,  52   e   3  that focuses the laser beam L onto the bottom surfaces  48   d  of the grindstones  48 . 
     The lens  52   e   4  is a cylindrical lens, for example. When the lens  52   e   4  focuses the laser beam L onto the bottom surfaces  48   d , the lens  52   e   4  shapes the laser beam L into a horizontally linear beam having a length commensurate with the width of each of the grindstones  48 , i.e., its dimension along the diameter of the grinding wheel  44 . The beam condenser  52   e   3  has a fixed relative position with respect to the grinding unit  36 . The laser beam L shaped into the horizontally linear beam is applied to the bottom surfaces  48   d  in such a manner as to extend across the bottom surfaces  48   d  in radial directions of the grinding wheel  44 , for example. The laser beam L focused as the horizontally linear beam on the bottom surfaces  48   d  is applied substantially uniformly to the bottom surfaces  48   d  in their entirety upon rotation of the grinding wheel  44  compared with a laser beam focused as a laser beam spot on the bottom surfaces  48   d.    
     The laser beam applying unit  52   e  illustrated in  FIG.  12    is fixed to the base  4 , and the beam condenser  52   e   3  is disposed directly below the grinding surface  48   a  outside of the relative movement area B. Laser processing conditions under which to process the workpiece  11  with the laser beam L emitted from the laser beam applying unit  52   e  are set as follows, for example: 
     Wavelength: 1030 nm 
     Repetitive frequency: 200 kHz 
     Pulse duration: 8 ps 
     Average output power: 30 W 
     According to the sixth embodiment, when the workpiece  11  is ground by the grinding wheel  44  in the creep feed grinding mode, the laser beam L emitted from the beam condenser  52   e   3  is applied to the bottom surfaces  48   d  of the grindstones  48 , thereby cleaning and/or correcting the bottom surfaces  48   d . Specifically, the applied laser beam L melts or vaporizes the binder of the grindstones  48 , grinding debris or swarf from the workpiece  11 , etc., and gives energy to the abrasive grains of the grindstones  48 . Thus, it is possible to at least remove grinding debris or swarf from the grindstones  48 , dress the grindstones  48 , or correct the shape of the grindstones  48  at the bottom surfaces  48   d  thereof. Consequently, a grindstone condition failure of the bottom surfaces  48   d  can be eliminated without a reduction in the efficiency of grinding in the creep feed grinding mode. 
     In  FIG.  12   , the single beam condenser  52   e   3  is disposed directly below the grinding surface  48   a  outside of the relative movement area B. However, two or more beam condensers  52   e   3  may be disposed directly below the grinding surface  48   a . Further, two or more beam condensers  52   e   3  may be disposed outside of the relative movement area B on one side or respective both sides thereof along the Y-axis. In particular, if two beam condensers  52   e   3  are disposed diametrically across the center  48   c  on the outside diameter  48   b  of the grinding surface  48   a , then as described above, the degree of freedom of the spindle  40  can be secured. The first modification or the second modification described above is also applicable to the creep feed grinding apparatus  62   e  according to the fifth embodiment. 
     The structure, method, etc., according to the above embodiments may be changed or modified appropriately without departing from the scope of the present invention. For example, the chuck table  6  may be of a rectangular plate shape rather than a circular plate shape. If the chuck table  6  is of a rectangular plate shape, then the holding surface  6   a  is a substantially flat rectangular surface. The workpiece  11  held under suction on the holding surface  6   a  is not limited to a circular plate wafer. The workpiece  11  may be a rectangular strip substrate including molded resin or the like. The workpiece  11  may be in a state of a frame unit including a plurality of strip substrates held on a frame ring by a protective tape, and each of the strip substrates may be ground by any of the creep feed grinding apparatuses according to the above embodiments in the creep feed grinding mode. 
     Different two of the bottom surface state adjusting units  50 ,  50   a ,  50   b ,  50   c ,  50   d , and  50   e  may be used in combination. For example, the bottom surface state adjusting units  50  and  50   c  may be combined with each other. In such a combination, the first nozzle  52  is disposed in one of two locations spaced diametrically across the center  48   c  on the outside diameter  48   b  of the grinding surface  48   a  and the dresser  52   c   1  is disposed in the other of the two locations. The high-pressure water  54  ejected from the first nozzle  52  and the dresser  52   c   1  clean and/or correct the bottom surfaces  48   d.    
     Particularly, the high-pressure water  54  is effective to remove grinding debris or swarf from the grindstones  48 . The dressing of the grindstones  48  with the dresser  52   c   1  is effective to dress and correct the shape of the grindstones  48 . Therefore, the first nozzle  52  may be disposed in one of the locations where the grindstones  48  leave the workpiece  11 , and the dresser  52   c   1  may be disposed in the other location where the grindstones  48  start to grind the workpiece  11 , as viewed in plan on the X-Y plane. 
     Alternatively, the bottom surface state adjusting units  50  and  50   d  may be combined with each other to clean and/or correct the bottom surfaces  48   d  with the high-pressure water  54  from the first nozzle  52  and the brush  52   d . Further alternatively, the bottom surface state adjusting units  50  and  50   e  may be combined with each other to clean and/or correct the bottom surfaces  48   d  with the high-pressure water  54  from the first nozzle  52  and the laser beam L. Combinations of two of the bottom surface state adjusting units  50 ,  50   a ,  50   b ,  50   c ,  50   d , and  50   e  other than the combinations described above may be used to clean and/or correct the bottom surfaces  48   d . In addition, different three of the bottom surface state adjusting units  50 ,  50   a ,  50   b ,  50   c ,  50   d , and  50   e  may be used in combination. 
     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.