Abstract:
In order not to transmit an impact when grinding is started, or micro-vibrations of a grinding wheel during grinding to a wafer, a grinding apparatus at least includes: a chuck table that holds a wafer; a grinding unit having a grinding wheel configured to include a grinding wheel part that is fixed to a wheel base and grinds a wafer held on the chuck table and having a wheel mount that supports the wheel base; and a grinding unit feeding unit that brings the grinding unit dose to and away from the chuck table, wherein a vibration damping rubber having a rebound resilience of 2% to 4% standardized by ISO 4662 is provided between the wheel base and the wheel mount, whereby an impact when grinding is started, or micro-vibrations of a grinding wheel during grinding are absorbed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a grinding apparatus that grinds a wafer and a method of grinding a wafer using the grinding apparatus. 
         [0003]    2. Prior Art 
         [0004]    A wafer having a plurality of devices such as an integrated circuit formed on the front surface thereof is divided into individual devices by a dicing apparatus, and the devices are used for various electronic appliances. 
         [0005]    Before the wafer is divided into individual devices, its back surface is ground to smooth the back surface, and the wafer is finished to have a desired thickness. For grinding the back surface of the wafer, the front surface side of the wafer is held on a chuck table of the grinding apparatus, and a rotating grinding wheel is brought into contact with the back surface of the wafer to apply a predetermined pressing force for grinding (for example, see JP-A-2007-222986). 
         [0006]    However, when the rotating grinding wheel is fed for grinding and brought into contact with the wafer and the strength reaches a predetermined pressing force to start grinding, a strong impact is transmitted to the wafer at the moment of the start. In addition, during grinding, microvibrations are generated in the grinding wheel because of grinding resistance, and the wafer is finely beaten correspondingly. Therefore, due to these phenomena, a problem arises that grinding distortion such as cracks occurs in the grinding surface of the wafer to deteriorate die strength of the devices configuring the wafer. 
       SUMMARY OF THE INVENTION 
       [0007]    An object to be solved by the invention is that in the case in which a grinding wheel is brought into contact with a wafer for grinding, an impact at the time of starting grinding caused by a grinding wheel, or microvibrations of a grinding wheel during grinding is prevented from being transmitted to the wafer. 
         [0008]    A first aspect of the invention is a grinding apparatus at least including: a chuck table that holds a wafer, a grinding unit having a grinding wheel configured to include a grinding wheel part that is fixed to a wheel base and grinds a wafer held on the chuck table and having a wheel mount that supports the wheel base; and a grinding unit feeding unit that brings the grinding unit dose to and away from the chuck table, wherein a vibration damping rubber having a rebound resilience of 2% to 4% standardized by ISO 4662 is provided between the wheel base and the wheel mount. 
         [0009]    A second aspect of the invention is a method of grinding a wafer in which the grinding apparatus according to the first aspect is used to grind a wafer, the method including the steps of holding a wafer on the chuck table as a back surface of the wafer is exposed; rotating the chuck table; and feeding the grinding unit for grinding by the grinding unit feeding unit while the grinding wheel is rotated, and bringing the grinding wheel into contact with the back surface of the wafer to grind the back surface. 
         [0010]    In the method of grinding a wafer, preferably, a feed speed of the grinding unit by the grinding unit feeding unit is 0.1 μm/sec. to 15 μm/sec., a rotating speed of the chuck table is 10 rpm to 400 rpm, and a rotating speed of the grinding wheel is 1000 rpm to 7200 rpm. In the case in which on a front surface of the wafer, a plurality of devices is formed as the devices are defined by streets as dividing lines, a protective member is bonded to on the front surface and the protective member is held on the chuck table. As an exemplary wafer, a silicon wafer is named. 
         [0011]    According to the invention, because a vibration damping rubber having a rebound resilience of 2% to 4% standardized by ISO 4662 is provided between the wheel base and the wheel mount of the grinding wheel, even though the grinding unit is fed for grinding to apply a predetermined pressing force to the wafer, the impact at this time is absorbed through the vibration damping rubber to prevent the impact from being transmitted to the wafer. In addition, because micro-vibrations caused by grinding resistance during grinding are also absorbed by the vibration damping rubber, finely beating the wafer is softened. Therefore, stripes are hardly formed on the grinding surface of the wafer to prevent die strength of the device from being deteriorated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view showing an exemplary grinding apparatus; 
           [0013]      FIG. 2  is an exploded perspective view showing a wheel mount, a vibration damping rubber and a grinding wheel configuring a grinding unit; 
           [0014]      FIG. 3  is a perspective view showing the state in which the wheel mount, the vibration damping rubber and the grinding wheel are fixed to each other; 
           [0015]      FIG. 4  is a perspective view showing an exemplary wafer of a grinding subject; 
           [0016]      FIG. 5  is a front view showing the state in which a protective member is bonded to the front surface of the wafer; and 
           [0017]      FIG. 6  is a cross section schematically showing the state before grinding the back surface of the wafer is started. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    In a grinding apparatus  1  shown in  FIG. 1 , a wafer cassette  100   a  that accommodates therein a wafer to be ground and a wafer cassette  100   b  that accommodates therein a wafer after ground are mounted, respectively on cassette mounting areas  10   a  and  10   b.    
         [0019]    Near the cassette mounting areas  10   a  and  10   b,  a carrying unit  11  that brings a wafer in and out of the wafer cassettes  100   a  and  100   b  is arranged. The wafer brought out of the wafer cassette  100   a  by the carrying unit  11  is placed on a positioning table  12 , and the wafer is positioned at a fixed position here. 
         [0020]    Near the positioning table  12 , a first carrying unit  13   a  is arranged. The first carrying unit  13   a  carries the wafer positioned on the positioning table  12  to any one of three chuck tables  14   a,    14   b,  and  14   c  that hold the wafer. Each of the chuck tables is configured of a holding part  140  that holds the wafer, and a holding part base  141  that rotatably supports the holding part  140 . In addition, these three chuck tables  14   a,    14   b,  and  14   c  are rotated in association with the rotation of a turntable  15 . 
         [0021]    In the grinding apparatus  1  shown in  FIG. 1 , a first grinding unit feeding unit  17  and a second grinding unit feeding unit  18  are arranged on a wall  16  erected from one end of the apparatus. The first grinding unit feeding unit  17  is configured of a pair of guide rails  170  vertically arranged, a ball screw  171  arranged in parallel with the guide rails  170 , a motor  172  joined to the tip end of the ball screw  171 , and an elevating part  173  that is slidably engaged with the guide rails  170  and has an internal nut screwed to the ball screw  171 , in which the elevating part  173  moves up and down as guided by the guide rails  170  in association with the rotation of the ball screw  171  driven by the motor  172 . 
         [0022]    The second grinding unit feeding unit  18  is configured of a pair of guide rails  180  vertically arranged, a ball screw  181  arranged in parallel with the guide rails  180 , a motor  182  joined to the tip end of the ball screw  181 , and an elevating part  183  that is slidably engaged with the guide rails  180  and has an internal nut screwed to the ball screw  181 , in which the elevating part  183  moves up and down as guided by the guide rails  180  in association with the rotation of the ball screw  181  driven by the motor  182 . 
         [0023]    The elevating part  173  configuring the first grinding unit feeding unit  17  supports a first grinding unit  19 . The first grinding unit  19  includes a spindle  200  having a vertical shaft, a spindle housing  190  that rotatably supports the spindle  200 , a wheel mount  210  formed at the tip end of the spindle  200 , a grinding wheel  230  supported by the wheel mount  210 , and a motor  200   a  that is joined to the spindle  200  to rotate the spindle  200 , in which the grinding wheel  230  is also rotated as the spindle  200  is driven by the motor  200   a  for rotation. The first grinding unit  19  is driven by the first grinding unit feeding unit  17 , and brought close to and away from the chuck table. 
         [0024]    The elevating part  183  configuring the second grinding unit feeding unit  18  supports a second grinding unit  20 . The second grinding unit  20  includes a spindle  201  having a vertical shaft, a spindle housing  191  that rotatably supports the spindle  201 , a wheel mount  211  formed at the tip end of the spindle  201 , a grinding wheel  231  supported by the wheel mount  211 , and a motor  201   a  that is joined to the spindle  201  to rotate the spindle  201 , in which the grinding wheel  231  is also rotated as the spindle  201  is driven by the motor  201   a  for rotation. The second grinding unit  20  is driven by the second grinding unit feeding unit  18 , and brought close to and away from the chuck table. 
         [0025]    As shown in  FIG. 2 , in the first grinding unit  19  and the second grinding unit  20 , the grinding wheel  230  ( 231 ) is configured in which grinding wheel parts  230   b  ( 231   b ) that grind a wafer is fixed in an arc shape to the bottom surface of a ring-shaped wheel base  230   a  ( 231   a ), and a plurality of screw holes  230   c  ( 231   c ) is formed in the wheel base  230   a  ( 231   a ). In addition, the grinding wheel part  230   b  is a grinding wheel of diamond abrasive grain having a grain size of about 10 μm fixed with a resin bond and the grinding wheel part  231   b  is a grinding wheel having a grain size of 1 μm or below of diamond abrasive grain fixed with a vitrified bond. 
         [0026]    For example, as shown in  FIG. 2 , in the wheel mount  210  ( 211 ), a plurality of through holes  210   a  ( 211   a ) is formed. On the wheel mount  210  ( 211 ), the grinding wheel  230  ( 231 ) is fixed and supported through a vibration damping rubber  220  ( 221 ). For example, for the vibration damping rubber  220  ( 221 ), “HANENITE” (registered trademark) provided by Kayo Corporation can be used. 
         [0027]    As shown in  FIG. 2 , a screw  240  ( 241 ) is inserted into the through hole  210   a  ( 211   a ) of the wheel mount  210  ( 211 ) and the through hole  220   a  ( 221   a ) of the vibration damping rubber  220  ( 221 ), and screwed to the screw hole  230   c  ( 231   c ) of the wheel base  230   a  ( 231   a ). Then, as shown in  FIG. 3 , the grinding wheel  230  ( 231 ) is fixed to the wheel mount  210  ( 211 ) through the vibration damping rubber  220  ( 221 ), and the wheel mount  210  ( 211 ) supports the wheel base  230   a  ( 231   a ) through the vibration damping rubber  220  ( 221 ). In addition, the vibration damping rubber  220  ( 221 ) may be fixed to the surface of the wheel base  230   a  ( 231   a ) on which the wheel mount  210  ( 211 ) is mounted and formed in one piece with the grinding wheel  230  ( 231 ) in advance. 
         [0028]    Next, a method of grinding a wafer will be described. For example, as shown in  FIG. 4 , in the case of grinding a back surface W 2  of a wafer W having a plurality of devices D formed on a front surface W 1  as defined by streets S, as shown in  FIG. 5 , a protective member P is bonded to the front surface W 1  to protect the devices, the wafer W is accommodated in the wafer cassette  100   a  shown in  FIG. 1  upside down, and the carrying unit  11  brings the wafer W out of the wafer cassette  100   a  to place it on the positioning table  12 . Then, after the wafer W is positioned at a fixed position, the first carrying unit  13   a  carries the wafer W to the chuck table  14   a,  for example. On the chuck table  14   a,  the surface having the protective member P bonded to the wafer W is held, and the back surface W 2  is exposed. 
         [0029]    Subsequently, the turntable  15  is rotated to position the wafer W directly below the grinding wheel  230  (the position of the chuck table  14   c  in  FIG. 1 ). Then, as shown in  FIG. 6 , the chuck table  14   a  is rotated as well as the first grinding unit  19  is fed for grinding by the first grinding unit feeding unit  17  (see  FIG. 1 ) while the grinding wheel part  230   b  is rotating in association with the rotation of the spindle  200 , and the grinding wheel part  230   b  is descended. The rotating grinding wheel part  230   b  is brought into contact with the back surface W 2  of the wafer W to grind the back surface W 2 . Here, for example, rough grinding is conducted. 
         [0030]    After rough grinding is finished, the turntable  15  is rotated to position the wafer W directly below the grinding wheel  231  (the position of the chuck table  14   b  in  FIG. 1 ). Then, as shown in  FIG. 6 , the chuck table  14   a  is rotated as well as the wheel mount  211  is fed for grinding by the second grinding unit feeding unit  18  (see  FIG. 1 ) while the grinding wheel part  231   b  is rotating in association with the rotation of the spindle  201 , and the grinding wheel part  231   b  is descended. The rotating grinding wheel part  231   b  is brought into contact with the back surface W 2  of the wafer W to grind the back surface W 2 . Here, finish grinding is conducted. 
         [0031]    Because the vibration damping rubber  220  ( 221 ) is provided between the grinding wheel  230  ( 231 ) and the wheel mount  210  ( 211 ), in starting rough grinding and finish grinding, an impact when the grinding wheel part  230   b  ( 231   b ) is brought into contact with the back surface W 2  of the wafer W is absorbed by the effect of the vibration damping rubber  220  ( 221 ), whereby the impact is not transmitted to the wafer W. In addition, the grinding wheel part  230   b  ( 231   b ) is brought into contact with the wafer W to generate grinding resistance to cause micro-vibrations in the grinding wheel parts  230   b  ( 231   b ). However, the microvibrations are also absorbed in the vibration damping rubber  220  ( 221 ), and hardly transmitted to the wafer W, and thus it is softened to beat the wafer W. Therefore, grinding distortion such as cracks hardly occurs in the grinding surface of the wafer W (the back surface W 2 ), and the deterioration of the die strength of the individual devices D configuring the wafer W and the breakage can be prevented. 
         [0032]    The wafer W thus ground is positioned near the second carrying unit  13   b  by the rotation of the turntable  15  shown in  FIG. 1 , and carried to a cleaning unit  25  by the second carrying unit  13   b.  Grinding wastes are removed here, and then the wafer W is accommodated in the wafer cassette  100   b  by the carrying unit  11 . 
         [0033]    In addition, in the embodiment above, the vibration damping rubber is provided to both of the first grinding unit  19  and the second grinding unit  20 . However, it may be configured to provide the vibration damping rubber to only one of them, for example, the second grinding unit  20 . In addition, the grinding apparatus  1  having two grinding wheels are taken and described as an example. However, the invention can be also adapted to such apparatuses having a single grinding wheel or three or more. Moreover, the number of chuck tables may be any numbers. 
       EXAMPLE 
       [0034]    As the vibration damping rubbers  220  and  221  shown in  FIG. 2 , a vibration damping rubber having a rebound resilience of 2%, 4%, 6%, 8%, 10%, and 12% standardized by ISO 4662 was provided between the wheel bases  230  and  231  and the wheel mounts  210  and  211  to grind the back surface of a silicon wafer. 
         [0035]    In grinding, the speeds were varied in the following ranges, where the feed speed of the first grinding unit  19  and the second grinding unit  20  was in the range of 0.1 μm/sec. to 15 μm/sec., the rotating speed of the chuck table was in the range of 10 rpm to 400 rpm, and the rotating speed of the grinding wheels  230  and  231  was in the range of 1000 rpm to 7200 rpm. 
         [0036]    In the case in which the vibration damping rubbers having rebound resilience of 6% or above were used, stripes were observed on the grinding surface of the wafer. These stripes are grinding distortion, which become the factor that deteriorates the die strength of the devices configuring the wafer. On the other hand, in the case in which the vibration damping rubbers having a rebound resilience of 2% and 4% were used, stripes were not observed particularly on the grinding surface of the wafer after ground which were caused by the second grinding unit  20 . Therefore, when the rebound resilience is set from 2% to 4%, the deterioration in the transverse strength of the device can be prevented.