Patent Publication Number: US-11376707-B2

Title: Grinding method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a grinding method for grinding a central portion of a reverse side of a workpiece to form in the reverse side of the workpiece a disk-shaped recess including a circular portion that has been ground and a ring-shaped stiffening portion that has been unground which surrounds the periphery of the circular ground portion. 
     Description of the Related Art 
     Workpieces having a plurality of areas demarcated on a face side thereof by a grid of projected dicing lines and a plurality of devices such as integrated circuits (ICs), or large-scale-integration (LSI) circuits formed in the respective areas are divided into a plurality of device chips by a grinding step, a cutting process, and so on. One method of grinding a workpiece grinds only a circular central portion of the reverse side of the workpiece that is aligned thicknesswise across the workpiece with a circular device region of the face side of the workpiece where a plurality of devices are disposed (see, for example, Japanese Patent Laid-open No. 2007-19461). When only the circular central portion of the reverse side of the workpiece is ground, there is formed in the reverse side a disk-shaped recess defined by a circular portion that has been ground and a ring-shaped stiffening portion that has been unground which surrounds the periphery of the circular ground portion. The ring-shaped stiffening portion that is left unground on the reverse side allows the workpiece to be handled, e.g., to be transported, with ease even though the reverse side of the workpiece has been thinned down. 
     SUMMARY OF THE INVENTION 
     However, in the grinding step, a number of chippings tend to be produced on the reverse side of the ring-shaped stiffening portion, lowering its own mechanical strength, due to an impact of grindstones contacting the reverse side of a workpiece and outer side faces of the grindstones contacting an inner circumferential side face of the ring-shaped stiffening portion. Furthermore, in a case where the workpiece is processed by wet etching after the grinding step, the areas of the ring-shaped stiffening portion where the chippings have been formed are etched, forming surface irregularities on the reverse side of the ring-shaped stiffening portion. The surface irregularities thus formed are likely to cause other problems in subsequent processes. For example, a metal film evaporated on the reverse side of the ring-shaped stiffening portion is liable to be peeled off from the surface irregularities that act as peel initiating points. Moreover, when a dicing tape is affixed to the reverse side of the workpiece, the dicing tape is likely to fail to be affixed properly to the workpiece on account of the surface irregularities. The present invention has been made in view of the above-described problems. It is an object of the present invention to provide a grinding method for grinding a workpiece while reducing the number of chippings produced on the reverse side of a ring-shaped stiffening portion of the workpiece. 
     In accordance with an aspect of the present invention, there is provided a grinding method of grinding a reverse side of a disk-shaped workpiece having on a face side thereof a device region where a plurality of devices are formed and an outer circumferential surplus region surrounding the device region, with a grinding stone part of a grinding wheel having an annular wheel base, the grinding stone part being disposed in an annular pattern on a surface of the wheel base. The grinding method includes a face side protecting step of covering the face side of the workpiece with a protective member, a holding step of holding the face side of the workpiece under suction on a disk-shaped chuck table that is rotatable about a central axis of a first rotational shaft, after the holding step, an oblique grinding step of rotating the grinding wheel about the central axis of a second rotational shaft, the grinding wheel being mounted on the lower end of a second rotational shaft, having a diameter smaller than the diameter of the chuck table, and disposed above the chuck table, tilting the second rotational shaft with respect to the first rotational shaft such that the bottom of a first portion of the grinding wheel that is positioned above an outer circumferential portion of the chuck table is higher than the bottom of a second portion of the grinding wheel that is positioned above a central portion of the chuck table, and then moving the grinding wheel and the chuck table relatively to each other to bring the grinding wheel and the chuck table closer to each other along a direction parallel to the first rotational shaft, thereby forming a disk-shaped recess in the reverse side of the workpiece by grinding a central portion of the reverse side of the workpiece that corresponds to the device region thicknesswise across the workpiece, the disk-shaped recess being defined by a circular ground portion and a ring-shaped stiffening portion surrounding the circular ground portion and left unground, and after the oblique grinding step, a tilt changing and grinding step of grinding the reverse side of the workpiece while gradually changing a tilt of the second rotational shaft to orient the second rotational shaft parallel to the first rotational shaft. 
     Preferably, the method may further include after the tilt changing and grinding step, an ordinary grinding step of orienting the second rotational shaft of the grinding wheel and the first rotational shaft of the chuck table parallel to each other and then moving the grinding wheel and the chuck table relatively to each other to bring the grinding wheel and the chuck table closer to each other along the direction parallel to the first rotational shaft, thereby grinding the circular ground portion. 
     According to the aspect of the present invention, in the oblique grinding step, the grinding wheel is rotated about the central axis of the second rotational shaft, the second rotational shaft is tilted with respect to the first rotational shaft such that the bottom of the first portion of the grinding wheel that is positioned above the outer circumferential portion of the chuck table is higher than the bottom of the second portion of the grinding wheel that is positioned above the central portion of the chuck table, and then the disk-shaped recess is formed in the reverse side of the workpiece by grinding the central portion of the reverse side of the workpiece. In this manner, the ring-shaped stiffening portion on the reverse side is prevented from producing chippings that would be otherwise formed due to contact between the outer side surfaces of the grindstones and the inner circumferential edge of the upper surface of the ring-shaped stiffening portion. Consequently, the number of chippings that may be formed on the reverse side of the ring-shaped stiffening portion is minimized. 
     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 side elevational view, partly in cross section, of a grinding apparatus on which a grinding method according to an embodiment of the present invention is carried out; 
         FIG. 2  is a perspective view of a workpiece with a protective member affixed thereto; 
         FIG. 3A  is a side elevational view, partly in cross section, of a workpiece unit placed on a chuck table; 
         FIG. 3B  is a side elevational view, partly in cross section, of a grinding wheel, the workpiece unit, and the chuck table in an oblique grinding step; 
         FIG. 4A  is a side elevational view, partly in cross section, of a grinding wheel, the workpiece unit, and the chuck table at the time the grinding wheel is grinding the workpiece; 
         FIG. 4B  is an enlarged fragmentary side elevational view, partly in cross section, of a portion of the assembly illustrated in  FIG. 4A ; 
         FIG. 5  is an enlarged fragmentary side elevational view, partly in cross section, illustrating a tilt changing and grinding step; 
         FIG. 6  is a side elevational view, partly in cross section, illustrating an ordinary grinding step; 
         FIG. 7  is a flowchart of the sequence of the grinding method; and 
         FIG. 8  is a side elevational view, partly in cross section, illustrating a grinding step according to a comparative example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A grinding method according to an embodiment of the present invention will be described in detail below with reference to the accompanying drawings. First, a grinding apparatus  2  on which the grinding method is carried out will be described below with reference to  FIG. 1 .  FIG. 1  is a side elevational view, partly in cross section, of the grinding apparatus  2 . As illustrated in  FIG. 1 , the grinding apparatus  2  has a base  4  substantially in the form of a rectangular parallelepiped supporting a plurality of components of the grinding apparatus  2  thereon. A disk-shaped chuck table  6  is rotatably mounted on the base  4 . The chuck table  6  has a frame  6   a  made of ceramic that has a fluid channel, not illustrated, defined therein. The fluid channel has an end connected to a suction source, not illustrated, such as an ejector. 
     The frame  6   a  has a recess defined as a disk-shaped space in an upper surface thereof. A disk-shaped porous plate  6   b  is fixedly disposed in the recess. Note that the diameter of the recess and the porous plate  6   b  is substantially the same as the diameter of a workpiece  11  (see  FIG. 2 ) to be described later, and may be 200 mm, for example. The fluid channel defined in the frame  6   a  has the other end connected to the porous plate  6   b . When the suction source is actuated, it generates and transmits a vacuum through the fluid channel and acts on the upper surface of the porous plate  6   b , which thus functions as a holding surface  6   c  for holding the workpiece  11  under suction thereon. 
     A first rotary actuator, not illustrated, such as an electric motor is disposed in the base  4  below the lower surface of the chuck table  6 . The first rotary actuator has an output shaft, i.e., a first rotational shaft,  8  extending substantially parallel to a Z-axis direction, i.e., a heightwise direction or vertical direction. The output shaft  8  is coupled to the lower surface of the chuck table  6 . When the first rotary actuator is energized, it rotates the output shaft  8  about its central axis, rotating the chuck table  6  about its central axis, i.e., the central axis of the output shaft  8 . A column  10  in the form of a rectangular parallelepiped is erected on a rear area of the base  4  behind the chuck table  6 . 
     A grinding feed unit  12  is disposed in a front portion of the column  10 . The grinding feed unit  12  has a pair of guide rails  12   a  extending substantially parallel to the heightwise direction and fixed to a front side surface of the column  10 . Note that, in  FIG. 1 , one of the guide rails  12   a  that is remoter from the viewer of  FIG. 1  is illustrated and the other guide rail  12   a  closer to the viewer is omitted from illustration. A movable plate  12   b  is slidably mounted on the guide rails  12   a . A nut  12   c  is mounted on a reverse side, i.e., a rear surface, of the movable plate  12   b  and operatively threaded over a ball screw  12   d  disposed in the column  10  and extending substantially parallel to the heightwise direction. The ball screw  12   d  is rotatable about its central axis. 
     The ball screw  12   d  has an upper end coupled to a stepping motor  12   e . When the stepping motor  12   e  is energized, it rotates the ball screw  12   d  about its central axis, causing the nut  12   c  to move the movable plate  12   b  along the guide rails  12   a . A grinding unit  14  includes a tubular holder  14   a  fixedly mounted on a face side, i.e., a front surface, of the movable plate  12   b . The holder  14   a  houses a tubular spindle housing  14   b  disposed in an inner space thereof. 
     An annular array of spacers  14   c  ( 14   c   1 ,  14   c   2 ) each shaped as a block is disposed on the lower surface of the spindle housing  14   b . Each of the spacers  14   c  has an upper surface held in contact with the lower surface of the spindle housing  14   b  and a lower surface disposed on the upper surface of a bottom plate of the holder  14   a . In  FIG. 1 , one of the spacers  14   c  that is disposed in a position closest to the column  10  is illustrated as a spacer  14   c   1 , and another one of the spacers  14   c  that is disposed in a position remotest from the column  10  is illustrated as a spacer  14   c   2 . The spacer  14   c   2  has an internally threaded hole defined in a lower portion thereof. 
     The bottom plate of the holder  14   a  has a through hole defined therein that is positioned below the internally threaded hole in the spacer  14   c   2  in alignment therewith. A screw  14   d  has an externally threaded shank operatively threaded through the through hole in the bottom plate of the holder  14   a  into the internally threaded hole in the spacer  14   c   2 . The screw  14   d  has a head exposed below the bottom plate of the holder  14   a  and coupled to the output shaft of an actuating mechanism, not illustrated, such as an electric motor. When the actuating mechanism is energized to rotate the screw  14   d  in one direction about its central axis, the spacer  14   c   2  moves upwardly, spacing its lower surface slightly away from the upper surface of the bottom plate of the holder  14   a . When the actuating mechanism is energized to rotate the screw  14   d  in the opposite direction about its central axis, the spacer  14   c   2  moves downwardly, bringing its lower surface into contact with the upper surface of the bottom plate of the holder  14   a . The thickness of the spacer  14   c   2 , the distance that the spacer  14   c   2  moves, etc. are appropriately adjusted to realize the grinding method for the workpiece  11  as described later. 
     The spindle housing  14   b  houses a cylindrical spindle, i.e., a second rotational shaft,  14   e  therein. The spindle  14   e  is rotatably supported in the spindle housing  14   b . The spindle  14   e  has an upper end coupled to a second rotary actuator, not illustrated, such as an electric motor, disposed in the spindle housing  14   b . The spindle  14   e  extends downwardly from the spindle housing  14   b  and through an opening defined centrally in the bottom plate of the holder  14   a . The spindle  14   e  has a lower end positioned below the lower surface of the bottom plate of the holder  14   a  and coupled to a central portion of the upper surface of a disk-shaped wheel mount  16 . 
     The wheel mount  16  has a lower surface on which the upper surface of an annular wheel base  18   a  made of aluminum alloy or the like is mounted. In other words, the wheel base  18   a  is mounted on the lower end of the spindle  14   e  through the wheel mount  16 . The wheel base  18   a  is disposed above the chuck table  6 . The diameter of the wheel base  18   a  is smaller than the diameter of the chuck table  6 . For example, the diameter of the wheel base  18   a  is set to a predetermined length smaller than the diameter of the holding surface  6   c  that is approximately 200 mm according to the present embodiment. 
     The wheel base  18   a  has a lower surface, i.e., a surface,  18   b  on which a plurality of grindstones  18   c  each shaped as a block, i.e., a grinding stone part, are disposed in an annular pattern referred to as a segment array. Alternatively, a single annular grindstone, i.e., a grinding stone part, rather than the plurality of grindstones  18   c  may be disposed on the lower surface of the wheel base  18   a  in a pattern referred to as a continuous array. The wheel base  18   a  and the grindstones  18   c  are jointly included in a grinding wheel  18 . When the second rotary actuator is energized, it rotates the spindle  14   e  about its central axis, rotating the grinding wheel  18  about the central axis of the spindle  14   e  as the second rotational shaft. The spindle  14   e  and the wheel base  18   a  have a fluid channel, not illustrated, defined therein for supplying a grinding fluid such as pure water therethrough to the grindstones  18   c . During a grinding step, the grinding fluid is supplied from a grinding fluid supply source, not illustrated, through the fluid channel to the grindstones  18   c.    
     Next, the workpiece  11  to be ground by the grinding unit  14  will be described below.  FIG. 2  illustrates the workpiece  11  and a protective member  19  in perspective. The workpiece  11  according to the present embodiment includes a disk-shaped wafer made of a semiconductor material such as silicon, for example. The workpiece  11  has a predetermined thickness ranging from 100 to 800 μm, for example. The workpiece  11  has a face side  11   a  having a plurality of areas demarcated by a grid of projected dicing lines or streets  13  and a plurality of devices  15  such as ICs, or LSI circuits, each of the plurality of devices  15  being formed in the respective areas. The workpiece  11  is not limited to any materials, shapes, structures, sizes, and so on. For example, a substrate made of a semiconductor material other than silicon may be used as the workpiece  11 . The devices  15  are similarly not limited to any kinds, numbers, shapes, structures, sizes, and so on. 
     On the face side  11   a  of the workpiece  11 , the devices  15  are disposed within a circular device region  17   a  thereof. The device region  17   a  is surrounded by an annular outer circumferential surplus region  17   b  in which the devices  15  are not disposed lying on the outer side of the device region  17   a . In  FIG. 2 , the boundary between the circular device region  17   a  and the annular outer circumferential surplus region  17   b  is indicated by a broken line. The boundary represents a hypothetical line and is not actually applied as a visible line to the workpiece  11 . The face side  11   a  and a reverse side  11   b  opposite the face side  11   a  have their outer circumferential edges whose corners are beveled as illustrated in  FIGS. 3A through 6 . 
     The grinding method for grinding the workpiece  11  will be described below with reference to  FIGS. 2 through 7 .  FIG. 7  is a flowchart of the sequence of the grinding method. First, as illustrated in  FIG. 2 , a circular protective member  19  made of resin is affixed to the face side  11   a  of the workpiece  11 . The workpiece  11  and the protective member  19  that covers the face side  11   a  thereof are jointly included in a workpiece unit  21  (a face side protecting step S 10  illustrated in  FIG. 7 ). The protective member  19  is affixed to the workpiece  11  in covering relation to the beveled corner of the outer circumferential edge of the face side  11   a  as well as the face side  11   a  itself. The protective member  19  includes a disk-shaped sheet of resin and has a base layer and a glue layer, i.e., an adhesive layer, disposed on one surface of the base layer. The glue layer is made of ultraviolet-curable resin, though it may be made of thermosetting rein or naturally curable resin. Note that the base layer may not necessarily include a glue layer. For example, the protective member  19  may have a base layer only and may be affixed to the face side  11   a  by thermocompression, for example. 
     After the face side protecting step S 10 , the holding surface  6   c  of the chuck table  6  holds the face side  11   a  of the workpiece  11 , or specifically the surface of the protective member  19  that is opposite the surface thereof affixed to the face side  11   a  of the workpiece  11 , under suction thereon (a holding step S 20 ).  FIG. 3A  is a side elevational view, partly in cross section, of the workpiece unit  21  placed on the chuck table  6 . After the holding step S 20 , the grinding wheel  18  of the grinding apparatus  2  grinds the reverse side  11   b  of the workpiece  11  on the chuck table  6 . Specifically, according to the present embodiment, first, the actuating mechanism that is coupled to the screw  14   d  is energized to rotate the screw  14   d  about its central axis for thereby adjusting the position of the spacer  14   c   2 . 
     As illustrated in  FIG. 3B , the lower end of the spindle  14   e  is positioned between an outer circumferential portion  6   c   1  of the holding surface  6   c  and a central portion  6   c   2  of the holding surface  6   c . The spacer  14   c   2  is positionally adjusted by the actuating mechanism to tilt the spindle  14   e  through a predetermined angle θ from the vertical direction toward the central portion  6   c   2  of the holding surface  6   c . The predetermined angle θ, represented by an arc degree, is in a range larger than 0 arc degree and equal to or smaller than 2 arc degrees, i.e., 0 arc degree &lt;θ≤2 arc degrees. As illustrated in  FIG. 3B , by tilting the spindle  14   e  whose central axis is indicated by the dot-and-dash line through the predetermined angle θ from a straight line extending in the vertical direction parallel to the output shaft  8  as indicated by the broken line, the bottom of a first portion  18   c   1  of the grindstones  18   c  that is positioned above the outer circumferential portion  6   c   1  of the holding surface  6   c  becomes slightly higher than the bottom of a second portion  18   c   2  of the grindstones  18   c  that is positioned above the central portion  6   c   2  of the holding surface  6   c . Then, the spindle  14   e  is rotated to rotate the grinding wheel  18  about the central axis of the spindle  14   e , and the output shaft  8  is rotated to rotate the chuck table  6  about the central axis of the output shaft  8 . For example, the spindle  14   e  is rotated at a rotational speed of 3000 rpm and the output shaft  8  is rotated at a rotational speed of 300 rpm. 
     Then, the stepping motor  12   e  is energized to move the grinding wheel  18  and the chuck table  6  relatively to each other in a direction parallel to the output shaft  8  in order to bring the grinding wheel  18  and the chuck table  6  closer to each other. For example, the grinding unit  14  is grounding-fed downwardly along the Z-axis direction at a speed of 1.0 μm/second. The grindstones  18   c  of the grinding wheel  18  are brought into grinding contact with the reverse side  11   b  of the workpiece  11 , thereby grinding the reverse side  11   b  (an oblique grinding step S 30 ).  FIG. 3B  is a side elevational view, partly in cross section, of the grinding wheel  18 , the workpiece unit  21 , and the chuck table  6  in the oblique grinding step S 30 . 
     When the grindstones  18   c  contact the reverse side  11   b  of the workpiece  11 , the grindstones  18   c  grind the reverse side  11   b  and removes part of the reverse side  11   b . According to the present embodiment, the grindstones  18   c  do not grind an outer circumferential portion  11   b   1  of the reverse side  11   b  that corresponds to the outer circumferential surplus region  17   b  but grind a central portion  11   b   2  of the reverse side  11   b  that corresponds to the device region  17   a  thicknesswise across the workpiece  11 .  FIG. 4A  is a side elevational view, partly in cross section, of the grinding wheel  18 , the workpiece unit  21 , and the chuck table  6  at the time the grinding wheel  18  is grinding the workpiece  11 . The central portion  11   b   2  is ground into a circular ground portion  11   c   2  by the grindstones  18   c . The ground portion  11   c   2  is surrounded by the outer circumferential portion  11   b   1  disposed therearound that is left unground as a ring-shaped stiffening portion  11   c   1 . 
     The ground portion  11   c   2  and the ring-shaped stiffening portion  11   c   1  that surrounds the ground portion  11   c   2  define a disk-shaped recess  11   c  centrally in the reverse side lib of the workpiece  11 .  FIG. 4B  is an enlarged fragmentary side elevational view, partly in cross section, of a portion of the assembly illustrated in  FIG. 4A  in the vicinity of the boundary between the ground portion  11   c   2  and the ring-shaped stiffening portion  11   c   1 . In the oblique grinding step S 30 , the spindle  14   e  is tilted the predetermined angle θ with respect to the output shaft  8 . Therefore, the grindstones  18   c  grind the reverse side lib while the grindstones  18   c  are having their outer side surfaces spaced from an inner circumferential edge of the upper surface of the ring-shaped stiffening portion  11   c   1 , i.e., the outer circumferential portion  11   b   1  of the reverse side lib. In other words, when the grindstones  18   c  grind the reverse side lib, a gap  23  is defined between the outer side surfaces of the grindstones  18   c  and the inner circumferential edge of the upper surface of the ring-shaped stiffening portion  11   c   1 . 
     For example, providing the ring-shaped stiffening portion  11   c   1  has an inner circumferential side surface that is 600 μm deep along the Z-axis direction, if 0=1.9 degrees, then the outer side surfaces of the grindstones  18   c  are spaced from the inner circumferential edge of the upper surface of the ring-shaped stiffening portion  11   c   1  by a distance of 20 μm. Consequently, in a case where the abrasive grains on the outer side surfaces of the grindstones  18   c  protrude therefrom by a distance of 10 μm, the abrasive grains on the outer side surfaces of the grindstones  18   c  are kept out of contact with the inner circumferential edge of the upper surface of the ring-shaped stiffening portion  11   c   1 . Since the gap  23  is thus defined in the oblique grinding step S 30  according to the present embodiment, the ring-shaped stiffening portion  11   c   1  on the reverse side  11   b  is prevented from producing chippings that would be otherwise formed due to contact between the outer side surfaces of the grindstones  18   c  and the inner circumferential edge of the upper surface of the ring-shaped stiffening portion  11   c   1 . Consequently, the number of chippings that may be formed on the reverse side of the ring-shaped stiffening portion  11   c   1  is minimized. 
     After the oblique grinding step S 30 , the grinding unit  14  stops being grinding-fed, and the actuating mechanism is energized to rotate the screw  14   d  in the opposite direction about its central axis, thereby gradually changing the tilt of the spindle  14   e  in a direction opposite the direction in which the spindle  14   e  is tilted in the oblique grinding step S 30 . According to the present embodiment, the tilt of the spindle  14   e  is adjusted in order to cancel out the predetermined angle θ formed in the oblique grinding step S 30 , orienting the spindle  14   e  parallel to the output shaft  8 . While the tilt of the spindle  14   e  is changing, the grindstones  18   c  continue to grind the reverse side lib of the workpiece  11  (a tilt changing and grinding step S 40 ). 
       FIG. 5  is an enlarged fragmentary side elevational view, partly in cross section, of a portion of the assembly in the vicinity of the boundary between the ground portion  11   c   2  and the ring-shaped stiffening portion  11   c   1 , illustrating the tilt changing and grinding step S 40 . Note that, in  FIG. 5 , the grindstones  18   c  that are tilted in the oblique grinding step S 30  are indicated by the two-dot-and-dash lines, whereas the grindstones  18   c  whose tilt is eliminated in the tilt changing and grinding step S 40  are indicated by the solid lines. Note that, in the tilt changing and grinding step S 40 , as the spindle  14   e  is brought parallel to the output shaft  8 , the ground portion  11   c   2  is made flatter than if the grinding step is finished in the oblique grinding step S 30 . In the tilt changing and grinding step S 40 , an annular curved surface  11   d  is formed in the vicinity of the boundary between the bottom of the inner circumferential side surface of the ring-shaped stiffening portion  11   c   1  and the ground portion  11   c   2 . 
     In the tilt changing and grinding step S 40  according to the present embodiment, the grinding unit  14  is not grinding-fed downwardly. However, the spindle  14   e  may be oriented parallel to the output shaft  8  while grinding-feeding the grinding unit  14  at a speed of 1.0 μm/second. After the tilt changing and grinding step S 40 , with the spindle  14   e  and the output shaft  8  lying parallel to each other, the ground portion  11   c   2  is further ground by the grindstones  18   c  (an ordinary grinding step S 50 ).  FIG. 6  is a side elevational view, partly in cross section, illustrating the ordinary grinding step S 50 . 
     In the ordinary grinding step S 50 , the grinding wheel  18  and the chuck table  6  are moved relatively to each other in order to bring the grinding wheel  18  and the chuck table  6  closer to each other in a direction parallel to the output shaft  8 . For example, the grinding unit  14  is grounding-fed downwardly at a speed of 1.0 μm/second. Note that, in the grinding method according to the present embodiment, the ordinary grinding step S 50  is not an indispensable step. Stated otherwise, the grinding of the workpiece  11  may be finished when the steps from the face side protecting step S 10  to the tilt changing and grinding step S 40  are carried out. 
     A comparative example will be described below.  FIG. 8  is a side elevational view, partly in cross section, illustrating a grinding step according to the comparative example. According to the comparative example, after the face side protecting step S 10  and the holding step S 20 , the spindle  14   e  is oriented parallel to the output shaft  8 , and then the spindle  14   e  and the output shaft  8  are rotated respectively about their central axes, and the grinding unit  14  grinds the reverse side  11   b  of the workpiece  11 . In this case, the outer side surfaces of the grindstones  18   c  are held in contact with the inner circumferential edge of the upper surface of the ring-shaped stiffening portion  11   c   1  as illustrated by a region  25  indicated by a dot-and-dash-line circle in  FIG. 8 . Consequently, more chippings are likely to be produced on the reverse side  11   b  of the ring-shaped stiffening portion  11   c   1 , compared with the embodiment described above. 
     The structure, process, and other details according to the present embodiment may be changed or modified within the scope of the present invention. For example, the grinding method according to the present embodiment is applicable to both rough grinding and finishing grinding. 
     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.