Patent Publication Number: US-6702652-B2

Title: Method of grinding rear side of semiconductor wafer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of grinding the rear side of a semiconductor wafer, on the front side of which bumps are formed. 
     2. Related Art 
     Referring to FIG. 11, a semiconductor wafer W 1  has ICs, LSIs or other circuits formed on its front side, and a protection tape T 1  is applied to the front side of the semiconductor wafer W 1 . The semiconductor wafer W 1  is put on a chuck table  50  with its front side down, and the semiconductor wafer W 1  is ground to a predetermined thickness by applying the grindstone  51  to its rear side and by rotating the grindstone  51 . 
     Referring to FIG. 12, a semiconductor wafer W 2  has terminals  52  (called “bumps”) formed on its front side. A protection tape T 2  whose adhesive layer is thick enough to bury the bumps  52 , is applied to the front side. Alternatively a protection tape coated with ultraviolet-sensitive glue may be attached to the front side of the semiconductor wafer W 2 , and the tape is exposed to ultraviolet rays prior to the grinding so that the glue may be set. Thus, the stress applied to each bump is reduced in grinding so that the semiconductor wafer W 2  may be prevented from cracking. 
     In either case the stress applied to the bumps  52  cannot be removed completely, and therefore, the cracking of semiconductor wafers cannot be prevented completely. Also, the necessity of using extra tapes as described above is economically disadvantageous. 
     It is, therefore, required that the cracking of semiconductor wafers having bumps formed on their front sides be completely prevented in grinding their rear sides without involving extra costs. 
     SUMMARY OF THE INVENTION 
     To meet such requirement a method of grinding the rear side of a semiconductor wafer according to the present invention comprises the steps of: preparing semiconductor wafers whose front surfaces have a plurality of circuits formed in lattice patterns; coating the front surface of a selected one of the semiconductor wafers with a resist material to form a resist layer thereon; forming a plurality of holes in the resist layer to extend through the resist layer to the front surface of the semiconductor wafer; forming a plurality of metal bumps to project as bumps from the front surface of the semiconductor wafer such that each of the metal bumps is disposed in one of the holes formed in the resist layer; positioning the semiconductor wafer, which has the metal bumps disposed in the holes of the resist layer, on a chuck table such that the resist layer is supported on the chuck table and the front surface of the semiconductor wafer faces the chuck table; and grinding a rear surface of the semiconductor wafer while the semiconductor wafer is positioned on the chuck table. 
     The method may further comprise the step of applying a protection tape to the resist layer at the front side of the semiconductor wafer such that, when the semiconductor wafer is positioned on the chuck table, the protection tape is in contact with the chuck table and supports the resist layer and the semiconductor wafer. 
     The bumps are lower than the thickness of the resist layer, and the bumps may be formed by plating at the holes with gold or a soldering metal, each bump being 50 to 200 μm in diameter, and 50 to 200 μm in height. 
    
    
     Other objects and advantages of the present invention will be understood from the following description of one preferred embodiment of the present invention, which is shown in the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a semiconductor wafer, on which bumps are to be formed; 
     FIG. 2 is a front view of the semiconductor wafer, one surface of which a resist layer is formed; 
     FIG. 3 illustrates, in section, how small holes are made in the resist layer of the semiconductor wafer; 
     FIG. 4 illustrates, in section, how bumps are made in the small holes; 
     FIG. 5 is a perspective view of the semiconductor wafer having the bumps formed thereon; 
     FIG. 6 shows the semiconductor wafer which has a protection tape applied to its resist layer to cover the bumps; 
     FIG. 7 is a perspective view of a grinding machine, which is used in grinding the rear sides of semiconductor wafers with bumps formed on their front sides; 
     FIG. 8 illustrates, in section, a semiconductor wafer laid on a selected chuck table in the grinding machine; 
     FIG. 9 is a perspective view of a grinding wheel having pieces of coarse or fine grindstone fixed to its lower side; 
     FIG. 10 illustrates, in section, a semiconductor wafer from which the resist layer is removed to expose the bumps; 
     FIG. 11 illustrates how the rear side of a bump-less semiconductor wafer is ground; and 
     FIG. 12 illustrates how the rear side of a semiconductor wafer having bumps formed on its front side is ground. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring to FIG. 1, a semiconductor wafer W has a plurality of crossing streets S formed on its front side, and each square section C has a circuit pattern formed therein. 
     As seen from FIG. 2, a resist material is coated over the whole surface of a semiconductor wafer W by a resist coater such as a spinning coater to form a resist layer  1  thereon. The resist thickness is formed larger than the height of bumps which are formed later. 
     With use of an aligner such as a stepping projection aligner, a number of minute holes  2  equal to the number of bumps to be formed later are made, as seen from FIG. 3, by removing the resist at the positions corresponding to the positions at which the bumps are to be formed, by exposure and developing. The diameter of the minute hole  2  is equal to that of the bump, which is to be made later. 
     Bumps  3  are made by plating at the minute holes  2  with a metal as shown in FIG.  4 . Specifically bumps  3  are made with gold or soldering metal, and each bump  3  is 50 to 200 μm in diameter, and 50 to 200 μm in height. In FIG. 5 all minute holes  2  are filled with bumps  3 . 
     Then, the rear side  4  of the semiconductor wafer W is ground without removing the resist layer  1 . The bumps  3  are short of reaching the upper surface of the resist layer  1 , thus allowing the rear side  4  of the semiconductor wafer W to be ground without applying a protection tape T to the front side of the semiconductor wafer on which the bumps  3  are formed. Thus, the semiconductor wafer of FIGS. 4 and 5 can be ground as it is. 
     In a case that a protection tape T is applied to the front side of a semiconductor wafer W, the protection tape T used need not be a special type having a thick adhesive layer, and an ordinary protection tape can be used as is the case with the grinding of a bump-less semiconductor wafer. The manner in which a semiconductor wafer having no protection tape applied to its front side is ground is described below. 
     In grinding the rear side of a semiconductor wafer W, a grinding machine  10  as shown in FIG. 7 can be used, and a plurality of semiconductor wafers W as shown in FIGS. 4 and 5 are put in a cassette  11 . 
     A putting in-and-taking out means  12  takes semiconductor wafers one by one to turn each semiconductor wafer W upside down and put it on a positioning means  13 . After the semiconductor wafer W is oriented there, a first transferring means  14  transfers the semiconductor wafer W to a selected chuck table  15  where the semiconductor wafer W is laid with its rear side  4  up, as seen from FIG.  8 . 
     The chuck tables  15 ,  16  and  17  are rotatably supported by a turntable  18 . The turntable  18  is rotated counterclockwise through predetermined angular intervals (120 degrees in the example of FIG. 7) to cause the semiconductor wafers W initially transferred onto a chuck table (e.g.  15 ) by the first transferring means  14  to move one after another to a position under a coarse grinding means  20 . 
     The coarse grinding means  20  is carried by a carrier  24 , which rides on a pair of vertical, parallel guide rails  22  laid on an upright wall  21 . The carrier  24  can be raised or lowered by a drive motor  23 , and accordingly the coarse grinding means  20  can be raised or lowered. The coarse grinding means  20  has a grinding wheel  27  attached to its spindle  25  via an associated mount  26 . The grinding wheel  27  comprises an annular body  28  and pieces of grindstone  29  attached to the bottom of the annular body  28 , as shown in FIG.  9 . 
     While rotating the spindle  25 , the grindstone  29  of the coarse grinding means  20  is lowered to be pushed against the rear side of the semiconductor wafer W, thereby effecting the coarse grinding on the semiconductor wafer W. 
     After finishing the coarse grinding, the turntable  18  is made to turn 120 degrees counterclockwise, and then, the coarse-ground wafer W is positioned under a fine grinding means  30 . 
     The fine grinding means  30  is carried by a carrier  33 , which rides on a pair of vertical, parallel guide rails  31  laid on the upright wall  21 . The carrier  33  can be raised or lowered by a drive motor  32 , and accordingly the fine grinding means  30  can be raised or lowered. The fine grinding means  30  has a grinding wheel  36  attached to its spindle  34  via an associated mount  35 . The grinding wheel  36  comprises an annular body  37  and pieces of fine grindstone  38  attached to the bottom of the annular body  37 , as shown in FIG.  9 . 
     While rotating the spindle  34 , the grindstone  38  of the fine grinding means  30  is lowered to be pushed against the rear side  4  of the semiconductor wafer W, thereby effecting the fine grinding on the semiconductor wafer W. 
     After finishing the fine grinding, the semiconductor wafer W is transferred to a washing means  41  by a second transporting means  40  to remove the debris from the semiconductor wafer W. The semiconductor wafer W thus cleaned is put in a cassette  42  by the putting in-and-taking out means  12 . 
     As described above, all semiconductor wafers are taken out of the cassette  11  to be coarse- and fine-ground sequentially, and the finished semiconductor wafers are put in the cassette  42 . 
     The rear side of each semiconductor wafer is ground while the bumps  3  are buried in the resist layer, thus preventing concentration of the stress to each bump, and permitting even distribution of the stress over the semiconductor wafer with the result that no cracking is caused in the semiconductor wafer. 
     Extra protection tapes having thick protection adhesive layers or ultraviolet-settable protection layers need not be used. This is advantageous to economy and productivity. The resist layer functions like a protection tape, and therefore the semiconductor wafer can be put on a chuck table with its resist layer facing the chuck table without the necessity of applying a protection tape to the semiconductor wafer. This will significantly improves the productivity. 
     All semiconductor wafers are taken out one by one from the cassette  42  to be transferred to a resist removing station where the resist layer  1  is removed from each semiconductor wafer, thus providing the semiconductor wafer W having its bumps  3  protruding from its front surface. 
     As may be understood from the above, the method of grinding the rear side of a semiconductor wafer according to the present invention provides the following advantages: 
     The grinding method permits the grinding of the rear side of the semiconductor wafer without removing the resist layer from its front surface, not allowing the stress to be concentrated to the bumps of the semiconductor wafer. Thus, the semiconductor wafer is guaranteed to be free of any cracking. Extra protection tapes need not be used, and accordingly the certainty, productivity and economy can be improved. 
     The resist layer functions like an extra type of protection tape, and therefore the semiconductor wafer can be put on a chuck table with its resist layer facing the chuck table without the necessity of applying an extra type of protection tape to the semiconductor wafer. This will significantly improve the economy and productivity. 
     In a case that a protection tape is applied to the resist layer, the resist layer can be removed along with the protection tape from the semiconductor wafer.