Patent Publication Number: US-10332777-B2

Title: Wafer processing method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a wafer processing method for use in grinding a wafer having asperities formed on the front side. 
     Description of the Related Art 
     There is an increasing chance of thinning a wafer before dividing the wafer into device chips, so as to reduce the size and weight of each device chip adapted to be incorporated into various electronic equipment or the like. For example, the wafer can be thinned by holding the front side of the wafer on which devices axe formed on a chuck table, rotating a grinding wheel, and pressing the grinding wheel against the back side of the wafer to thereby grind the back side of the wafer. 
     In grinding the wafer by using the above method, a protective member is usually attached to the front side of the wafer (see Japanese Patent Laid-Open Ho. Hei 10-50642, for example), The protective member can prevent the possibility that the devices formed on the front side of the wafer may he damaged by a force applied to the wafer in grinding the wafer. Examples of the protective member include an adhesive tape formed of resin and a substrate having high hardness. 
     SUMMARY OF THE INVENTION 
     In many cases, asperities such as bumps functioning as electrodes on each device are formed on the front side of the wafer. However, when such asperities are present on the front side of the wafer, there arises a problem such that a height difference due to the asperities cannot be sufficiently absorbed by an adhesive tape and the shape corresponding to each asperity may appear on the back side of the wafer after grinding. 
     By using a substrate having high hardness as the protective member, the above problem can be almost eliminated. However, this substrate is bonded to the wafer by using an adhesive such as a thermoplastic wax. Accordingly, in peeling the substrate from the wafer after grinding, it is necessary to conduct any extensive work dedicated to peeling, such as immersion of the wafer in a solution or heating of the wafer at high temperatures. 
     It is therefore an object of the present invention to provide a wafer processing method which can sufficiently suppress the influence of the asperities present on the front side of the wafer in grinding the back side of the wafer and can simply perform a peeling operation after grinding. 
     In accordance with an aspect of the present invention, there is provided a wafer processing method including a liquid supplying step of supplying a liquid to the front side of a wafer on which a central device area and a peripheral marginal area surrounding the device area are formed, a plurality of devices each having asperities being formed in the device area; a close contact making step of pressing a protective film against the front side of the wafer with the liquid interposed therebetween after performing the liquid supplying step, thereby bringing the protective film into close contact with the front side of the wafer so as to follow the asperities of each device formed on the front side of the wafer; a protective member fixing step of covering the protective film with a protective member formed from a liquid resin curable by external stimulus after performing the close contact making step, thereby fixing the protective member through the protective film to the front side of the wafer; a grinding step of holding the protective member fixed to the wafer on a holding surface of a chuck table in the condition where the back side of the wafer is exposed after performing the protective member fixing step, and next grinding the back side of the wafer to thereby reduce the thickness of the wafer; and a peeling step of peeling the protective film and the protective member from the wafer after performing the grinding step. 
     Preferably, the peeling step includes the step of heating the liquid left between the protective film and the front side of the wafer to thereby vaporize the liquid. 
     Preferably, the peeling step includes the steps of loading the wafer with the protective member into a vacuum chamber and next evacuating the vacuum chamber to thereby vaporize the liquid left between, the protective film and the front side of the wafer. 
     Preferably, the liquid supplying step includes the step of supplying the liquid to a central portion of the front side of the wafer. 
     Preferably, the protective film has a first surface and a second surface opposite to the first surface, the first surface being opposed to the wafer; the close contact making step including the step of discharging a gas toward the second surface of the protective film to thereby press the protective film against the front side of the wafer. 
     Preferably, the close contact making step includes the step of using a pressure roller to thereby press the protective film against the front side of the wafer. 
     Preferably, the protective member fixing step includes the steps of applying the liquid resin to a flat sheet, next pressing the wafer against the liquid resin through the protective film to thereby fully cover the protective film with the liquid resin, and next applying the external stimulus to the liquid resin to thereby cure the liquid resin, thereby forming the protective member from the liquid resin and fixing the protective member to the wafer. 
     In the wafer processing method according to the present invention, the protective film is first brought into close contact with the front side of the wafer on which the devices each having asperities are formed in such a manner as to follow the asperities. Thereafter, the protective film is covered with the protective member formed from the liquid resin curable by external stimulus, thereby fixing the protective member through the protective film to the front side of the wafer. Accordingly, by forming the protective member having a suitable thickness, the asperities formed on the front side of the wafer can be sufficiently absorbed. 
     In the wafer processing method according to the present invention, the protective film is not bonded to the device area of the wafer, but is merely in close contact with the device area. Accordingly, the protective member and the protective film can be simply peeled from the wafer without the need for any extensive work dedicated to peeling, such as immersion of the wafer in a solution or heating of the wafer at high temperatures. Thus, according to the present invention, it is possible to provide a wafer processing method which can exhibit the effects that the influence of the asperities present on the front side of the wafer can be sufficiently suppressed in grinding the back side of the wafer and that the peeling operation after grinding can also be simply performed. 
     Further, in the wafer processing method, according to the present invention, the protective film is pressed against the front side of the wafer through the liquid supplied to the front side of the wafer. Accordingly, it is possible to prevent that air may be left between the wafer and the protective film, thereby reliably bringing the protective film into close contact with the front side of the wafer. As a result, although the protective film does not have adhesion by an adhesive (paste), there is no possibility that the protective film and the protective member may be peeled from the wafer in grinding the wafer. 
     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. 1A  is a schematic perspective view showing a manner of opposing a protective film to the front side of a wafer; 
         FIG. 1B  is a schematic perspective view showing a condition where the protective film is in close contact with the front side of the wafer; 
         FIG. 2A  is a schematic sectional view showing a condition where the protective film is opposed to the front side of the wafer in a close contact making step according to a preferred embodiment; 
         FIG. 2B  is a schematic sectional view showing a manner of bringing the protective film into close contact with the front side of the wafer in the close contact making step; 
         FIG. 3A  is a schematic sectional view showing a condition where the protective film has been brought into close contact with the front side of the wafer; 
         FIG. 3B  is an enlarged sectional view of a part of the wafer in the condition where the protective film is in close contact with the front side of the wafer; 
         FIG. 4A  is a schematic sectional view showing a manner of pressing the wafer through the protective film against a liquid resin applied to a sheet in a protective member fixing step according to this preferred embodiment; 
         FIG. 4B  is a schematic sectional view showing a manner of curing the liquid resin to thereby form a protective member, thereby fixing the protective member through the protective film to the front side of the wafer in the protective member fixing step; 
         FIG. 4C  is a schematic sectional view showing the wafer in the condition where the protective member supported to the sheet is fixed through the protective film to the front side of the wafer; 
         FIG. 5A  is a schematic side view, partially in cross section, showing a manner of grinding the back side of the wafer in a grinding step according to this preferred embodiment; 
         FIG. 5B  is a schematic sectional view of the wafer processed by the grinding step; 
         FIG. 6  is a schematic sectional view showing a manner of peeling the protective film and the protective member from the wafer in a peeling step according to this preferred embodiment; 
         FIG. 7  is a schematic sectional view showing a manner of peeling the protective film and the protective member from the wafer in a peeling step according to a first modification; 
         FIG. 8  is a schematic sectional view showing a manner of peeling the protective film and the protective member from the wafer in a peeling step according to a second modification; and 
         FIGS. 9A and 9B  are schematic sectional views for illustrating a close contact making step according to a third modification. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will now be described with reference to the attached drawings. The wafer processing method according to this preferred embodiment includes a liquid supplying step, a close contact making step (see  FIGS. 1A, 1B, 2A, 2B, 3A , and  3 B), a protective member fixing step (see  FIGS. 4A, 4B, and 4C ), and a grinding step (see  FIGS. 5A and 5B ), and a peeling step (see  FIG. 6 ). In the liquid supplying step, a liquid is supplied to the front side of a wafer on which asperities are formed. In the close contact making step, a protective film not having adhesion by an adhesive (paste) is brought into close contact with the front side of the wafer so as to follow the asperities formed on the front side of the wafer. In the protective member fixing step, the protective film is covered with a protective member formed from a liquid resin to thereby fix the protective member through the protective film, to the front side of the wafer. In the grinding step, the back side of the wafer is ground in the condition where the protective member fixed through the protective film to the front side of the wafer is held on a holding surface of a chuck table. In the peeling step, the protective member and the protective film are peeled together from the wafer thinned by the grinding step. The wafer processing method according to this preferred embodiment will now be described in more detail. 
     In the wafer processing method according to this preferred embodiment, the liquid supplying step is first performed to supply a liquid to the front side of a wafer on which asperities are formed. Thereafter, the close contact making step is performed to bring a protective film not having adhesion by an adhesive into close contact with the front side of the wafer so as to follow the asperities formed on the front side of the wafer. In other words, the protective film is pressed against the front side of the wafer with the liquid interposed therebetween, thereby bringing the protective film into close contact with the front side of the wafer.  FIG. 1A  is a schematic perspective view showing a manner of opposing the protective film to the front side of the wafer, and  FIG. 1B  is a schematic perspective view showing a condition where the protective film is in close contact with the front side of the wafer.  FIG. 2A  is a schematic sectional view showing a condition where the protective film is opposed to the front side of the wafer, and  FIG. 2B  is a schematic sectional view showing a manner of bringing the protective film into close contact with the front side of the wafer.  FIG. 3A  is a schematic sectional view showing a condition where the protective film has been brought into close contact with the front side of the wafer, and  FIG. 3B  is an enlarged sectional view of a part of the wafer in the condition where the protective film is in close contact with the front side of the wafer. In  FIGS. 2A, 2B, and 3A , a part of the components is shown by functional blocks. 
     Referring to  FIG. 1A , a wafer  11  to be used in this preferred embodiment is shown. The wafer  11  is a disk-shaped wafer formed of silicon (Si), for example. The wafer  11  has a front side  11   a , a back side  11   b , and an outer circumference  11   c . The outer circumference  11   c  is chamfered along the edges both on the front side  11   a  and on the back side  11   b . The front side  11   a  of the wafer  11  is composed of a central device area  11   d  and a peripheral marginal area  11   e  surrounding the device area  11   d . The device area  11   d  is partitioned by a plurality of crossing division lines (streets)  13  to thereby define a plurality of separate regions where a plurality of devices  15  such as ICs (integrated circuits) are individually formed. Further, a plurality of bumps (asperities)  17  functioning as electrodes are provided on the front side of each device  15 . Each bump  17  is formed of solder, for example. While the wafer  11  is a disk-shaped wafer formed of silicon, for example, in this preferred embodiment, the wafer  11  is not limited in material, shape, structure, size, etc. That is, the wafer  11  may be a wafer formed of any other semiconductors, ceramic, resin, metal, etc. Similarly, the devices  15  and the bumps  17  are not limited in kind, number, shape, structure, size, layout, etc. The bumps  17  may be replaced by any structures (asperities) having any other functions. That is, the bumps  17  may not be formed on the front side  11   a  of the wafer  11 , but any other asperities may be formed on the front side  11   a  of the wafer  11 . 
     In the liquid supplying step, a liquid  23  (see  FIG. 2A ) is supplied to the front side  11   a  of the wafer  11 . The liquid  23  to be supplied to the wafer  11  is not especially limited in kind, but it is desirable to use a liquid hard to vaporize at room temperature (20° C.) and having a boiling point not so high (e.g., 100° C. or less). For example, water may be used as the liquid  23 . In this preferred embodiment, the liquid  23  is supplied to the center of the front side  11   a  of the wafer  11 . As a modification, the liquid  23  may be supplied to another part of the front side  11   a  of the wafer  11  (i.e., any part other than the center of the front side  11   a ) or may be supplied to the whole of the front side  11   a  of the wafer  11 . 
     After performing the liquid supplying step, the close contact making step is performed to bring a protective film  21  into close contact with the front side  11   a  of the wafer  11  with the liquid  23  interposed therebetween. The protective film  21  is a flexible film formed of resin, for example. The protective film  21  is a circular member having a diameter substantially equal to that of the wafer  11 . Further, the protective film  21  is not provided with an adhesive. The thickness of the protective film  21  is not especially limited. For example, the thickness of the protective film  21  is preferably set to approximately 30 μm to 150 μm. 
     The close contact making step may be performed by using a close contact making apparatus  2  shown in  FIG. 2A . The close contact making apparatus  2  includes a support table  4  for supporting the wafer  11 . The support table  4  has a substantially flat upper surface, which, functions as a support surface  4   a  for supporting the wafer  11 . The support surface  4   a  is provided, with a projecting guide portion  4   b  for positioning the wafer  11  on the support surface  4   a . There is provided above the support table  4  a protective film holding unit  6  for holding the protective film  21  under suction and bringing it into close contact with the wafer  11 . The protective film holding unit  6  has a substantially flat lower surface, which functions as a holding surface  6   a  for holding the protective film  21  under suction. The protective film holding unit  6  is vertically movably supported by a moving mechanism (not shown), so that the protective film holding unit  6  holding the protective film  21  under suction can be moved in a vertical direction by operating this moving mechanism. 
     A first passage  6   b  is formed inside the protective film holding unit  6 , and one end of the first passage  6   b  opens to the holding surface  6   a  in a peripheral area for holding the peripheral portion of the protective film  21 . The other end of the first passage  6   b  is branched into a plurality of portions, more specifically, a first branch portion, a second branch portion, and a third branch portion. A vacuum source  10  is connected through a valve  8  to the first branch portion of the first passage  6   b . A compressed air source  14  is connected through a valve  12  to the second branch portion of the first passage  6   b . Further, a second passage  6   c  is also formed inside the protective film holding unit  6 , and one end of the second passage  6   c  opens to the holding surface  6   a  in a central area for holding the central portion of the protective film  21 . The other end of the second passage  6   c  is also branched into a plurality of portions, more specifically, a first branch portion and a second branch portion. The first branch portion of the second passage  6   c  is connected through a valve  16  to the third branch portion of the first passage  6   b . The compressed air source  14  is connected through a valve  18  to the second branch portion of the second passage  6   c . Further, a heater  20  for heating the holding surface  6   a  is provided inside the protective film holding unit  6 . 
     In the close contact making step, the wafer  11  is first placed on the support table  4  in the condition where the back, side  11   b  of the wafer  11  is in contact with the support surface  4   a  of the support table  4  as shown in  FIG. 2A . Accordingly, the wafer  11  is supported on the support table  4  in the condition where the front side  11   a  of the wafer  11  is exposed upward. Thereafter, the protective film  21  is brought into contact with the holding surface  6   a  of the protective film holding unit  6  so as to be aligned with the wafer  11 . More specifically, the outer circumference of the protective film  21  is positioned directly above the outer circumference  11   c  of the wafer  11 . In this condition, the valves  8  and  16  are opened to apply a vacuum generated from the vacuum source  10  to the protective film  21 . Accordingly, the protective film  21  is held on the holding surface  6   a  of the protective film holding unit  6  under suction so as to be opposed to the front side  11   a  of the wafer  11 . Before opening the valves  8  and  16 , the valves  12  and  18  are closed to stop the supply of compressed air (gas) from the compressed air source  14  to the first passage  6   b  and the second passage  6   c . The vertical position of the protective film holding unit  6  is adjusted so that the spacing between the front side  11   a  of the wafer  11  and the protective film  21  becomes approximately 0.1 to 10 mm, for example. 
     After opposing the protective film  21  to the front side  11   a  of the wafer  11 , the holding surface  6   a  is heated by the heater  20  to thereby apply heat to the protective film  21 , thereby softening the protective film  21 . Thereafter, as shown in  FIG. 2B , the valve  16  is closed to cut off the vacuum from the vacuum source  10  to the second passage  6   c , and the valve  18  is next opened to supply the compressed air from the compressed air source  14  to the second passage  6   c . Accordingly, the compressed air is discharged toward the central portion of the upper surface of the protective film  21 . In which the central portion of the upper surface of the protective film  21  is merely in contact with the holding surface  6   a  at this time (the upper surface of the protective film  21  corresponds to the second surface opposite to the first surface of the protective film  21  opposed to the wafer  11 ). As a result, the central portion of the protective film  21  is swelled downward and pressed against the front side  11   a  of the wafer  11  as shown in  FIG. 28 . In this preferred embodiment, the liquid  23  is previously supplied to the central portion of the front side  11   a  of the wafer  11  in the liquid supplying step as mentioned above, so that the protective film  21  is pressed against the front side  11   a  of the wafer  11  with a film of the liquid  23  interposed therebetween. 
     Thereafter, the discharge of the compressed air toward the protective film  21  is continued, so that the protective film  21  is pressed against the front side  11   a  of the wafer  11  in a radially outward direction starting from the center of the wafer  11 . After pressing the protective film  21  against the front side  11   a  of the wafer  11  in the whole area except the peripheral area yet held under suction, the valve  8  is closed to cut off the vacuum from the vacuum source  10  to the first passage  6   b , and the valve  12  is next opened to supply the compressed air from the compressed air source  14  to the first passage  6   b  as shown in  FIG. 3A . Accordingly, the compressed air is also discharged toward the peripheral portion of the upper surface of the protective film  21 , so that the peripheral portion of the protective film  21  is also pressed against the front side  11   a  of the wafer  11 . 
     As a result, the protective film  21  can be brought into close contact with the front side  11   a  of the wafer  11  so as to follow the shape of each bump  17  formed on the front side  11   a  of the wafer  11  as shown in  FIGS. 1B, 3A, and 3B . Further, in this preferred embodiment, the protective film  21  is heated to be softened by the heater  20 , so that the protective film  21  can be brought into close contact with the front side  11   a  of the wafer  11  so as to be properly fitted to each bump  17 . Further, when the protective film  21  is pressed against the front side  11   a  of the wafer  11 , the liquid  23  is moved by the protective film  21  to push out the air left between the protective film  21  and the front side  11   a  of the wafer  11 . Accordingly, the protective film  21  can be reliably brought into close contact with the front side  11   a  of the wafer  11  so as to prevent the possibility that the air may be left between the wafer  11  and the protective film  21 . In this preferred, embodiment, a film of the liquid  23  is left between the protective film  21 , and the wafer  11 . That is, the protective film  21  is brought into close contact with the front side  11   a  of the wafer  11  with a film of the liquid  23  interposed therebetween. 
     After performing the close contact making step, the protective member fixing step is performed to cover the protective film  21  with a protective member formed from a liquid resin, thereby fixing the protective member through the protective film  21  to the front side  11   a  of the wafer  11 .  FIG. 4A  is a schematic sectional view showing a manner of pressing the wafer  11  through the protective film  21  against a liquid resin applied to a sheet,  FIG. 4B  is a schematic sectional view showing a manner of curing the liquid resin to thereby form the protective member, thereby fixing the protective member through the protective film  21  to the front side  11   a  of the wafer  11 , and  FIG. 4C  is a schematic sectional view showing the wafer  11  in the condition where the protective member supported to the sheet is fixed through the protective film  21  to the front side  11   a  of the wafer  11 . In  FIGS. 4A and 4B , a part of the components is shown by functional blocks. 
     The protective member fixing step may be performed by using a protective member fixing apparatus  22  shown in  FIGS. 4A and 4B . The protective member fixing apparatus  22  includes a holding table  24  for holding a substantially flat sheet (carrier sheet)  25  formed of resin, for example. The holding table  24  has an upper surface formed with a central circular recess  24   a  larger in diameter than the wafer  11 . An ultraviolet light source  26  is provided in the recess  24   a . The upper end opening of the recess  24   a  is covered with a plate  28  capable of transmitting at least a part of ultraviolet, light applied from, the ultraviolet light source  26 . The sheet  25  is supported at its central portion by the plate  28 . A suction passage  24   b  is formed inside the holding table  24  in a peripheral area surrounding the recess  24   a , and one end of the suction passage  24   b  opens to the upper surface of the holding table  24  in this peripheral area, so as to hold a peripheral portion of the sheet  25  under suction. 
     The other end of the suction passage  24   b  is connected through a valve  30  to a vacuum source  32 . Accordingly, by applying a vacuum generated from the vacuum source  32  through the suction passage  24   b  to the peripheral portion of the sheet  25 , the sheet  25  can be held on the holding table  24  under suction. There is provided above the holding table  24  a wafer holding unit  34  for holding the wafer  11 . The wafer holding unit  34  has a lower surface  34   a  for holding the wafer  11 . The wafer holding unit  34  is vertically movably supported by a moving mechanism (not shown), so that the wafer holding unit  34  holding the wafer  11  can be moved in a vertical direction by operating this moving mechanism. The wafer holding unit  34  may be a vacuum suction type such that a vacuum is used to hold the wafer  11  under suction or may be of an electrostatic attraction type such that an electrostatic force is used to hold the wafer  11 . 
     In the protective member fixing step, a liquid resin  27  is previously applied to the upper surface of the sheet  25  as shown in  FIG. 4A , and the lower surface of the sheet  25  is held on the holding table  24 . Further, the back side  11   b  of the wafer  11  is held on the lower surface  34   a  of the wafer holding unit  34 . Accordingly, the protective film  21  kept in close contact with the front side  11   a  of the wafer  11  is opposed to the liquid resin  27  applied to the sheet  25 . The liquid resin  27  is a liquid resin curable by the ultraviolet light applied from the ultraviolet light source  26 . For example, TEMPLOC (registered trademark) manufactured by Denka Co., Ltd. may be used as the liquid resin  27 . While the sheet  25  is held on the holding table  24  in the condition where the liquid resin  27  is previously applied to the upper-surface of the sheet  25  in this preferred embodiment, the sheet  25  only may be first held on the holding table  24 , and the liquid resin  27  may be next applied to the upper surface of the sheet  25 . As shown in  FIG. 4A , the liquid resin  27  is not completely flat on the sheet  25 , but the central portion of the liquid resin  27  is preferably slightly raised. 
     Thereafter, as shown in  FIG. 4B , the wafer holding unit  34  is lowered to press the front side  11   a  of the wafer  11  through the protective film  21  against the liquid resin  27 . As a result, the liquid resin  27  is spread in the radial direction of the wafer  11  so as to fully cover the protective film  21 . In this preferred embodiment, the amount of the liquid resin  21  to be applied and the amount of lowering of the wafer holding unit  34  are adjusted so that the whole of the front side  11   a  of the wafer  11  is covered with the liquid resin  27 . Thereafter, ultraviolet light is applied from the ultraviolet light source  26  toward the liquid resin  27  to thereby cure the liquid resin  27 . Accordingly, as shown in  FIG. 4C , the liquid resin  27  is formed into a protective member  29  fully covering the protective film  21 , and this protective member  29  is fixed to the front side  11   a  of the wafer  11 . Thusly, the protective film  21  on the front side  11   a  of the wafer  11  is fully covered with the protective member  29  formed from the liquid resin  27 , thereby fixing the protective member  29  through the protective film  21  to the front side  11   a  of the wafer  11 . Further, the sheet  25  is fixed to the protective member  29 . 
     After performing the protective member fixing step, the grinding step is performed to grind the back side  11   b  of the wafer  11 .  FIG. 5A  is a schematic side view, partially in cross section, showing a manner of grinding the back side  11   b  of the wafer  11 , and  FIG. 5B  is a schematic sectional view of the wafer  11  processed by the grinding step. 
     The grinding step may be performed by using a grinding apparatus  42  shown in  FIG. 5A . The grinding apparatus  42  includes a holding table (chuck table)  44  for holding the wafer  11  under suction. The holding table  44  is connected to a rotational drive source (not shown) such as a motor. Accordingly, the holding table  44  is adapted to be rotated about its axis substantially parallel to a vertical direction, by operating this rotational drive source. A moving mechanism (not shown) is provided below the holding table  44 , so as to move the holding table  44  in a horizontal direction. The holding table  44  has an upper surface, a part of which is formed as a holding surface  44   a  for holding under suction the sheet  25  fixed through the protective member  29  to the wafer  11 . The holding surface  44   a  is connected through a suction passage (not shown) to a vacuum source (not shown), in which the suction passage is formed inside the holding table  44 . Accordingly, by applying a vacuum generated from the vacuum source through the suction passage to the holding surface  44   a  in the condition where the sheet  25  is in contact with the holding surface  44   a , the wafer  11  can be held through the sheet  25  and the protective member  29  on the holding table  44  under suction. 
     A grinding unit  46  is provided, above the holding table  44 . The grinding unit  46  includes a spindle housing (not shown) supported to an elevating mechanism (not shown). A spindle  48  is rotatably supported in the spindle housing. A disk-shaped mount  50  is fixed to the lower end of the spindle  48 . A grinding wheel  52  is mounted on the lower surface of the mount  50 , in which the grinding wheel  52  has substantially the same diameter as that of the mount  50 . The grinding wheel  52  includes a wheel, base  54  formed of metal such as stainless steel and aluminum. A plurality of abrasive members  56  are fixed to the lower surface of the wheel base  54  so as to be annularly arranged along the outer circumference of the wheel base  54 . A rotational drive source (not shown) such as a motor is connected to the upper end (base end) of the spindle  48 . Accordingly, the grinding wheel  52  fixed to the spindle  48  is adapted to be rotated about its axis substantially parallel to a vertical direction, by operating this rotational drive source to produce a rotational force. A nozzle (not shown) for supplying a grinding fluid such as pure water to the wafer  11  is provided inside or in the vicinity of the grinding unit  46 . 
     In the grinding step, the wafer  11  is first held on the holding table  44  of the grinding apparatus  42  under suction. More specifically, the wafer  11  is first placed on the holding table  44   a  of the holding table  44  in the condition where the sheet  25  fixed through the protective member  29  to the wafer  11  is in contact with the holding surface  44   a . That is, the back side  11   b  of the wafer  11  is exposed upward in this condition. Thereafter, the vacuum source is operated to apply a vacuum to the holding surface  44   a  of the holding table  44 . Accordingly, the wafer  11  is held through the sheet  25  and the protective member  29  on the holding table  44  under suction in the condition where the back side  11   b  of the wafer  11  is exposed upward. Thereafter, the holding table  44  is moved to the position below the grinding unit  46 . In this condition, both the holding table  44  and the grinding wheel  52  are rotated and the spindle housing (the spindle  48  and the grinding wheel  52 ) is then lowered as supplying the grinding fluid to the back side  11   b  of the wafer  11  as shown in  FIG. 5A . 
     The lowering speed (feed speed) of the spindle housing is adjusted so that the lower surface of each abrasive member  56  is pressed against the back side lib of the wafer  11  by a suitable force. Accordingly, the back side lib of the wafer  11  can be ground by the grinding wheel  52  to thereby reduce the thickness of the wafer  11 . When the thickness of the wafer  11  is reduced to a predetermined thickness (finished thickness) as shown in  FIG. 5B , this grinding step is finished. While one set of grinding unit  46  is used to grind the back side lib of the wafer  11  in this preferred embodiment, two or more sets of grinding units may be used to grind the back side  11   b  of the wafer  11 . For example, a first set of abrasive members each containing large-sized abrasive grains may be first used to perform coarse grinding on the back side  11   b  of the wafer  11 , and a second set of abrasive members each containing small-sized abrasive grains may be next used to perform fine grinding on the back side  11   b  of the wafer  11 . In this case, the flatness of the back side lib of the wafer  11  can be improved without greatly increasing the time required for grinding. 
     After performing the grinding step, the peeling step is performed to peel the protective film  21  and the protective member  29  from the wafer  11  thinned by the grinding step.  FIG. 6  is a schematic sectional view showing a manner of peeling the protective film  21  and the protective member  29  from the wafer  11 . 
     In the peeling step, a wafer holding unit  62  having a holding surface  62   a  is used to hold the wafer  11 . That is, the back side  11   b  of the wafer  11  is held on the holding surface  62   a  of the wafer holding unit  62 . The wafer holding unit  62  may be of a vacuum suction type such that a vacuum is used to hold the wafer  11  on the holding surface  62   a  under suction or may be of an electrostatic attraction type such that an electrostatic force is used to hold the wafer  11  on the holding surface  62   a . After holding the wafer  11  on the holding surface  62   a  of the wafer holding unit  62 , a peeling unit  64  is used to peel the protective member  29  and the protective film  21 . More specifically, a peripheral portion of the sheet  25  is gripped by the peeling unit  64 . Thereafter, the wafer holding unit  62  and the peeling unit  64  are relatively moved so that the peripheral portion of the sheet  25  is moved away from the wafer  11  as shown in  FIG. 6 . Accordingly, all of the protective film  21 , the protective member  29 , and the sheet  25  can be peeled together from the wafer  11  as shown in  FIG. 6 . 
     In the wafer processing method according to the above preferred embodiment mentioned above, the protective film  21  is first brought into close contact with the front side  11   a  of the wafer  11  on which the devices  15  each having the bumps (asperities)  17  are formed in such a manner as to follow the shape of each bump  17 . Thereafter, the protective film  21  is covered with the protective member  29  formed from the liquid resin  27  curable by ultraviolet light (external stimulus), thereby fixing the protective member  29  through the protective film  21  to the front side  11   a  of the wafer  11 . Accordingly, by forming the protective member  29  having a suitable thickness, the asperities due to the bumps  17  formed on the front side  11   a  of the wafer  11  can be sufficiently absorbed. 
     In the wafer processing method according to this preferred embodiment, the protective film  21  is not bonded to the device area lid of the wafer  11 , but is merely in close contact with the device area  11   d . Accordingly, the protective member  29  and the protective film  21  can be simply peeled from the wafer  11  without the need for any extensive work dedicated to peeling, such as immersion of the wafer  11  in a solution or heating of the wafer  11  at high temperatures. Thus, according to this preferred embodiment, it is possible to provide a wafer processing method which can exhibit the effects that the influence of the asperities due to the bumps  17  present, on the front side  11   a  of the wafer  11  can be sufficiently suppressed in grinding the back side lib of the wafer  11  and that the peeling operation after grinding can also be simply performed. 
     Further, in the wafer processing method according to this preferred embodiment, the protective film  21  is pressed against the front side  11   a  of the wafer  11  through the liquid  23  supplied, to the front side  11   a  of the wafer  11 . Accordingly, it is possible to prevent that air may be left between the wafer  11  and the protective film  21 , thereby reliably bringing the protective film  21  into close contact with the front side  11   a  of the wafer  11 . As a result, although the protective film  21  does not have adhesion by an adhesive (paste), there is no possibility that the protective film  21 , and the protective member  29  may be peeled from the wafer  11  in grinding the wafer  11 . 
     The present invention is not limited to the above preferred embodiment, but various modifications may be made. For example, while the liquid resin  27  is a liquid resin curable by ultraviolet light as external stimulus in this preferred embodiment, another type of liquid resin curable by any external stimulus (e.g., heat) other than ultraviolet light may be used as the liquid resin  27 . 
     Further, while the circular protective film  21  having a diameter substantially equal to that of the wafer  11  is used in this preferred embodiment, the diameter of the protective film  21  may be smaller than the diameter of the wafer  11 . For example, the diameter of the protective film  21  may correspond to the diameter of the device area lid of the wafer  11 . In this case, the protective member  29  formed from the liquid resin  27  adheres directly to the peripheral marginal area lie of the wafer  11 . Accordingly, although the protective film  21  does not have adhesion by an adhesive (paste), the protective film  21  and the protective member  29  can be firmly fixed to the wafer  11 . 
     Further, in this preferred embodiment, the wafer  11  is pressed through the protective film  21  against the liquid resin  27  applied to the sheet  25 , thereby fixing the protective member  29  to the wafer  11 . As a modification, the liquid resin  27  may be dropped onto the wafer  11  or the protective film  21  without using the sheet  25 , thereby fixing the protective member  29  to the wafer  11 . In this case, the exposed surface of the protective member  29  is preferably flattened by using a surface planer or the like. By flattening the exposed surface of the protective member  29  to be held on the holding table  44  in grinding the wafer  11 , the back side  11   b  of the wafer  11  as a work surface can be ground flat in the grinding step. 
     Further, while the protective film  21  is softened by the heat, applied from the heater  20  in the close contact making step in this preferred embodiment, softening of the protective film  21  is not essential in the close contact making step. Further, the protective film  21  may be softened by any methods other than heating by the heater  20 . 
     Further, while all of the protective film  21 , the protective member  29 , and the sheet  25  are peeled together from the wafer  11  by simply gripping the peripheral portion of the sheet  25  and moving it away from the wafer  11  in the peeling step according to this preferred embodiment, another method may be used to peel the protective film  21 , the protective member  29 , and the sheet  25  from the wafer  11 .  FIG. 7  is a schematic sectional view showing a manner of peeling the protective film  21 , the protective member  29 , and the sheet  25  from the wafer  11  in a peeling step in a wafer processing method according to a first modification. The other steps in the wafer processing method according to the first modification may be the same as those in the wafer processing method according to the above preferred embodiment. 
     In the peeling step according to the first modification, the back side  11   b  of the wafer  11  is held on the holding surface  62   a  of the wafer holding unit  62  as shown in  FIG. 7 . The wafer holding unit  62  may be of a vacuum suction type such that a vacuum is used to hold the wafer  11  on the holding surface  62   a  under suction or may be of an electrostatic attraction type such that an electrostatic force is used to hold the wafer  11  on the holding surface  62   a . In the first modification, a heater  66  is provided inside the wafer holding unit  62  as shown in  FIG. 7 . 
     After holding the wafer  11  on the holding surface  62   a  of the wafer holding unit  62 , a peripheral portion of the sheet  25  is gripped by the peeling unit  64 . Thereafter, the wafer holding unit  62  and the peeling unit  64  are relatively moved so that the peripheral portion of the sheet  25  is moved away from the wafer  11  as shown in  FIG. 7 . At this time, the heater  66  is operated to heat the liquid  23  left between the protective film  21  and the wafer  11 , thereby vaporizing the liquid  23 . Thus, all of the protective film  21 , the protective member  29 , and the sheet  25  can be peeled together from the wafer  11  as shown in  FIG. 7 . 
     In the wafer processing method according to the first modification, the liquid  23  left between the protective film  21  and the wafer  11  is heated to be vaporized by operating the heater  66 , so that the protective film  21  can be easily peeled from the wafer  11 . 
       FIG. 8  is a schematic sectional view showing a manner of peeling the protective film  21 , the protective member  29 , and the sheet  25  from the wafer  11  in a peeling step in a wafer processing method according to a second modification. The other steps in the wafer processing method according to the second modification may be the same as those in the wafer processing method according to the above preferred embodiment. 
     The peeling step according to the second modification may be performed by using a vacuum chamber  72  shown in  FIG. 8 . The vacuum chamber  72  includes a casing member  72   a  having an upper opening having a size allowing the pass of the wafer  11  and a door member  72   b  for closing the upper opening of the casing member  72   a . The casing member  72   a  is connected through an outlet pipe  74  and a valve  76  to a vacuum source (not shown). The casing member  72   a  is also connected to an inlet pipe  78  and a valve  80  for introducing the outside air (atmospheric air) into the vacuum chamber  72 . 
     The door member  72   b  is formed with a vertically extending through hole  72   c . A wafer holding unit  84  is inserted through the through hole  72   c  with a hermetic bearing  82  interposed therebetween, so that the wafer holding unit  84  is vertically movably supported to the door member  72   b  by the hermetic bearing  82 . The wafer holding unit  84  has a substantially flat lower surface  84   a  as a holding surface larger in diameter than the wafer  11 . The wafer holding unit  84  may be of an electrostatic attraction, type such that an electrostatic force is used to hold the wafer  11  on the lower surface  84   a . Further, a heater  86  is provided inside the wafer holding unit  84 . A peeling unit  88  is provided in the vicinity of the wafer holding unit  84 . 
     In the peeling step according to the second modification, the back side  11   b  of the wafer  11  is held on the lower surface  84   a  of the wafer holding unit  84 . Thereafter, the door member  72   b  is closed to cover the upper opening of the casing member  72   a , and the valve  80  is closed. Thereafter, the valve  76  is opened to thereby evacuate the inside space of the vacuum chamber  72 . Thereafter, a peripheral portion of the sheet  25  is gripped by the peeling unit  88 , and the wafer holding unit  84  and the peeling unit  88  are relatively moved so that the peripheral portion of the sheet  25  is moved away from the wafer  11 . At this time, the heater  86  may be operated to heat the liquid  23  left between the protective film  21  and the wafer  11 , thereby vaporizing the liquid  23 . Thus, all of the protective film  21 , the protective member  29 , and the sheet  25  can be peeled together from the wafer  11  as shown in  FIG. 8 . 
     In the wafer processing method according to the second modification, the protective film  21  is peeled from the wafer  11  under vacuum. Accordingly, the liquid  23  left between the protective film  21  and the wafer  11  is vaporized in the peeling step, so that the protective film  21  can be easily peeled. 
     Further, while the compressed, air is discharged toward the upper surface of the protective film  21  to thereby press the protective film  21  against the front side  11   a  of the wafer  11  in the above preferred embodiment, another method may be used to press the protective film  21  against the wafer  11 .  FIGS. 9A and 9B  are schematic sectional views for illustrating a close contact making step in a wafer processing method according to a third modification. 
     The wafer processing method according to the third modification includes a liquid supplying step, in which the liquid  23  is preferably supplied to at least an area where the close contact between the protective film  21  arid the wafer  11  is started. Accordingly, in the next close contact making step, the protective film  21  can be reliably brought into close contact with the front side  11   a  of the wafer  11  so as to prevent the possibility that air may be left between the wafer  11  and the protective film  21 . The other steps in the wafer processing method according to the third modification may be the same as those in the wafer processing method according to the above preferred embodiment or the above modifications. 
     In the close contact making step according to the third modification, the back side  11   b  of the wafer  11  is held on a holding surface  92   a  of a holding table  92 . The basic configuration of the holding table  92  may be the same as that of the holding table  44  except that a heater  94  is provided inside the holding table  92  in the third modification. After holding the wafer  11  on the holding table  92 , the protective film  21  previously held on the lower surface of a release sheet  31  is opposed to the front side  11   a  of the wafer  11 . Thereafter, a pressure roller  96  is used to apply a downward pressure to the upper surface of the release sheet  31 , thereby pressing the protective film  21  against the front side  11   a  of the wafer  11  as shown in  FIG. 9A . At this time, the heater  94  is preferably operated to heat the protective film  21 , thereby softening the protective film  21 . Accordingly, the protective film  21  can be brought into close contact with the front side  11   a  of the wafer  11  with the liquid  23  interposed therebetween. Finally, the release sheet  31  is released from the protective film  21  as shown in  FIG. 9B . As a modification, this close contact making step using the pressure roller  96  may be performed in a vacuum chamber. 
     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.