Abstract:
A grinding machine includes a holding table adapted to hold a workpiece, a grinding unit operative to grind the workpiece held on the holding piece, and a grinding unit transfer mechanism operative to shift the grinding unit in a direction coming close to or moving away from the workpiece. The grinding unit includes a porous pad having fine pores opposed to the workpiece, a gel-like slurry storing portion provided on the porous pad so as to store a gel-like slurry therein, and a water supply unit to supply water between the porous pad and the workpiece. The porous pad contains relatively larger superabrasives at least at an outer circumferential portion. The fine pores of the porous pad have a diameter greater than that of relatively small superabrasives contained in the gel-like slurry.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a grinding machine for grinding a workpiece such as a semiconductor wafer, a glass substrate, a ceramic substrate, etc. and a grinding method using the grinding machine. 
     2. Description of the Related Art 
     A semiconductor wafer, a glass substrate, a sapphire substrate, or a ceramic substrate such as SiC on which a plurality of devices such as ICs, LSIs or the like are formed is ground from a rear surface to a predetermined thickness. Thereafter, it is divided by a cutting machine such as a dicing machine or laser dicing machine into individual pieces, which are used in electronic devices such as cell phones, personal computers, etc. In general, a grinding wheel containing superabrasives such as diamond grindstones are widely used to grind the semiconductor wafer or the like. However, since the grinding by the grinding wheel is executed through fine brittle fracture, there arises a problem in that a ground surface causes a plurality of fine strains, which lowers bending strength of the workpiece. 
     To eliminate the problem, the ground surfaced of the workpiece after rough grinding is finish-ground and the finish-ground workpiece is subjected to finish processing such as polishing or etching to remove the strains generated by the grinding. As the finish grinding for a workpiece, a method is proposed for allowing gel-like slurry to flow out from fine pores of a porous pad and finish grinding the workpiece with superabrasives contained in the gel-like slurry. Incidentally, the gel-like slurry is a mixture of glue and superabrasives. If the porous pad gets wet, the gel-like slurry flows out from the porous pad to the upper surface of the workpiece. Thus, the workpiece is finish-ground with the superabrasives contained in the gel-like slurry. 
     A grinding machine combining the porous pad with gel-like slurry is effective in finish-grinding a workpiece. However, another grinding device is needed to rough-grind the workpiece with a rotatably driven grinding wheel. This is less preferable in view of economic efficiency. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a compact grinding machine that can alone perform rough grinding and finish grinding. 
     In accordance with an aspect of the present invention, there is provided a grinding machine including: a holding table adapted to hold a workpiece; grinding means for grinding the workpiece held on the holding table; and a grinding means transfer mechanism operative to move the grinding means in a direction coming close to or moving away from the workpiece; wherein the grinding means includes, a porous pad having a large number of fine pores opposed to the workpiece held on the holding table, a gel-like slurry storing portion provided on the porous pad so as to store gel-like slurry therein, and water supply means for supplying water between the porous pad and the workpiece; and the porous pad contains superabrasives at least at an outer circumferential portion, and the fine pores of the porous pad each have a diameter greater than that of each of the superabrasives contained in the gel-like slurry. 
     In accordance with another aspect of the present invention, there is provided a grinding method for grinding a workpiece using the above-mentioned grinding machine, including the steps of: grinding-transferring, by use of the grinding means transfer means, the porous pad in a direction towards the workpiece held on the holding table, and pressing the porous pad against the workpiece for performing rough grinding on the workpiece; and then, reducing pressing force of the porous pad against the workpiece and performing finish grinding on the workpiece with the gel-like slurry flowing out from the porous pad. 
     According to the present invention, the grinding means transfer mechanism grinding-transfers the porous pad and presses against the workpiece at a given pressure and the porous pad containing superabrasives is made to function as a grinding wheel for rough grinding. Thereafter, the pressing force is reduced to allow the gel-like slurry to flow out from the porous pad. The finish grinding of the workpiece can be performed with the superabrasives contained in the gel-like slurry. Thus, the grinding machine of the present invention can achieve both the rough grinding and finish grinding by means of the single grinding means without the necessity of a rough grinding portion and a finish grinding portion separate from each other. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exterior perspective view of a grinding machine according to an embodiment of the present invention; 
         FIGS. 2A and 2B  illustrate an attachment structure of a porous pad; 
         FIG. 3A  is a cross-sectional view illustrating a state of rough grinding; 
         FIG. 3B  is a cross-sectional view illustrating a state where the porous pad is lifted; 
         FIG. 4A  is a cross-sectional view illustrating a porous pad encountered during rough grinding according to another embodiment; 
         FIG. 4B  is a cross-sectional view illustrating the porous pad encountered during finish grinding according to the another embodiment; 
         FIG. 5A  is a cross-sectional view illustrating a porous pad encountered during rough grinding according to further another embodiment; and 
         FIG. 5B  is a cross-sectional view illustrating the porous pad encountered during finish grinding according to the further another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will hereinafter be described in detail with reference to the drawings.  FIG. 1  is a perspective view of a grinding machine  2  according to an embodiment of the present invention. Reference numeral  4  denotes a housing of the grinding machine  2 . A column  6  is provided at a rear portion of the housing  4  so as to extend upright. A pair of guide rails  8  are secured to the column  6  so as to extend upward and downward. (Only one of the guide rails  8  is illustrated.) A grinding unit (grinding means)  10  is mounted along the guide rails  8  so as to be shiftable upward and downward. The grinding unit  10  is such that its housing  20  is attached to a shifting base  12  which is shifted upward and downward along the pair of guide rails  8 . 
     The grinding unit  10  includes the housing  20 ; a spindle, not shown, rotatably housed in the housing  20 ; a servo motor  22  adapted to rotatably drive the spindle; a mount  24  secured to the leading end of the spindle; a gel-like slurry storing portion  25  attached to the mount  24 ; and a porous pad  26  disposed on the bottom of the gel-like slurry storing portion  25 . The grinding unit  10  includes a grinding unit shifting mechanism  18  composed of a ball screw  14  adapted to shift the grinding unit  10  upward and downward along the guide rails  8  and a pulse motor  16 . The pulse motor  16  is pulse-driven to rotate the ball screw  14  to shift the shifting base  12  upward and downward. 
     A holding table mechanism  28  having a holding table  50  is disposed at an intermediate portion of the housing  4 . The holding table mechanism  28  is shifted in a Y-axial direction by a holding table shifting mechanism not illustrated. Reference numeral  30  denotes a bellows covering the holding table mechanism  28 . A first wafer cassette  32 , a second wafer cassette  34 , a wafer-conveying robot  36 , a positioning mechanism  38  having a plurality of positioning pins  40 , a wafer carrying-in mechanism (loading arm)  42 , a wafer carrying-out mechanism (unloading arm)  44  and a spinner unit  46  are arranged at a front portion of the housing  4 . 
     A cleaning water jet nozzle  48  used to clean the holding table mechanism  28  is provided at a generally central portion of the housing  4 . In the state where the holding table mechanism  28  is positioned at a wafer-carrying-in and out area in front of the unit, the cleaning water jet nozzle  48  jets cleaning water toward the holding table mechanism  28 . Reference numeral  52  denotes a grinding water supply nozzle, which is adapted to jet water from a water jet port  53  located at its tip toward between the porous pad  26  and the wafer held on the holding table  50 . 
     Referring to  FIG. 2A , the mount  24  is secured to the leading end of the spindle  21  and the gel-like slurry storing portion  25  is attached to the mount  24 . The porous pad  26  is attached to the bottom of the gel-like slurry storing portion  25 .  FIG. 2B  illustrates another embodiment of a porous pad attachment structure, in which a porous pad  26   a  is attached to the bottom of a gel-like slurry storing portion  25  so as to block the bottom part thereof. 
       FIG. 3A  is a cross-sectional view illustrating a state where a wafer  54  held on the holding table  50  is rough-ground. Superabrasives  56  including diamond abrasive grains, CBN (cubic boron nitride) abrasive grains, or other abrasive grains are dispersed and arranged at the outer circumferential portion of the porous pad  26 . Gel-like slurry  58  including a mixture of glue material and fine superabrasives is stored in the gel-like slurry storing portion  25 . To rough-grind the wafer  54 , the wafer  54  is rotated in a given direction by the holding table  50 . Along with this, while the spindle  21  is rotated in the same direction as the wafer  54 , the grinding unit shifting mechanism  18  is driven to lower the porous pad  26  and press it against the wafer  54  on the holding table  50 . Thus, the rough grinding of the wafer  54  is performed while jetting grinding water from the grinding water supply nozzle  52 . 
     In this case, the porous pad  26  is pressed against the upper surface of the wafer  54  at a given pressure so that the superabrasives  56  in the porous pad  26  are exposed from the lower surface of the porous pad  26 . Thus, the rough grinding of the wafer  54  is performed by the exposed superabrasives  56 . After the rough grinding of the wafer  54  has been completed, the grinding unit shifting mechanism  18  is driven to slightly lift the porous pad  26  as illustrated in  FIG. 3B . Since grinding water is supplied to the porous pad  26  from the grinding water supply nozzle  52 , the gel-like slurry  58  containing fine abrasive grains  60  flows out to the upper surface of the wafer  54  via fine pores of the porous pad  26 . 
     If the pressing force against the porous pad  26  is released or the pressing force against the wafer  54  is small, the expansion of the porous pad  26  allows the superabrasives  56  not to almost go out from the front surface of the porous pad  26  but to be stored therein. Thus, if the grinding unit shifting mechanism  18  is driven to press the porous pad  26  against the wafer  54  at low pressure, the finish grinding of the wafer  54  can be performed by the fine superabrasives in the gel-like slurry  58  flowing out via the fine pores of the porous pad  26 . 
     Incidentally,  FIG. 3B  illustrates the state where the porous pad  26  is spaced apart from the wafer  54  in order to show the fine superabrasives  60  flowing out via the fine pores of the porous pad  26 . However, in practice, the pressing force against the porous pad  26  is reduced from the state of the rough grinding shown in  FIG. 3A  to withdraw the superabrasives  56  inwardly of the porous pad  26 . Subsequently to the rough grinding, finish grinding can be performed with the fine superabrasives  60  in the gel-like slurry  58  flowing out via the fine pores of the porous pad  26 . 
     Referring to  FIGS. 4A and 4B , another embodiment of a porous pad  26 ′ is illustrated in which superabrasives  56  are dispersed and arranged in the entire area thereof.  FIG. 4A  is a cross-sectional view illustrating a state during rough grinding and  FIG. 4B  is a cross-sectional view illustrating a state during finish grinding. During the rough grinding illustrated in  FIG. 4A , the porous pad  26 ′ is pressed against a wafer  54  at a given pressing force to expose the superabrasives  56  in the porous pad  26 ′ to the lower surface of the porous pad  26 ′. The rough grinding can be performed on the wafer  54  with the exposed superabrasives  56 . On the other hand, during the finish grinding of the wafer  54 , the pressing force of the porous pad  26 ′ against the wafer  54  is reduced to withdraw the superabrasives  56  inward of the porous pad  26 ′ as illustrated in  FIG. 4B . Thus, the finish grinding can be performed on the wafer  54  with the fine superabrasives  60  in the gel-like slurry  58  flowing out to the upper surface of the wafer  54  via the fine pores of the porous pad  26 ′. 
     Referring to  FIGS. 5A and 5B , a porous pad  26   a  according to another embodiment of the present invention is illustrated. The porous pad  26   a  is attached to a porous pad storing portion  25  so as to block the bottom thereof.  FIG. 5A  illustrates a state of the porous pad  26   a  encountered during rough grinding and  FIG. 5B  illustrates a state of the porous pad  26   a  encountered during finish grinding. 
     Also in the case of the porous pad  26   a  of the embodiment, operation encountered during grinding is the same as that of the embodiment described above. The gel-like slurry  58  may be supplied on the wafer  54  also during the rough grinding. Alternatively, the gel-like slurry  58  may not be supplied during the rough grinding but may supplied to the upper surface of the wafer also during the finish grinding. 
     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.