Patent Publication Number: US-9412637-B2

Title: Device wafer processing method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device wafer processing method of processing a device wafer having a plurality of devices formed on the front side, each device having an adhesion disliking region disliking the adhesion to an adhesive tape. 
     2. Description of the Related Art 
     In a semiconductor device fabrication process, a plurality of crossing division lines called streets are formed on the front side of a substantially disk-shaped semiconductor wafer such as a silicon wafer and a gallium arsenide wafer to thereby define a plurality of separate regions where a plurality of devices such as ICs and LSIs are respectively formed. 
     The back side of such a semiconductor wafer is ground by a grinding apparatus to reduce the thickness of the semiconductor wafer to a predetermined thickness. Thereafter, the semiconductor wafer is divided into individual device chips by a cutting apparatus or a laser processing apparatus. These device chips are widely used in various electronic equipment such as mobile phones and personal computers. Prior to grinding the back side of the semiconductor wafer, a protective tape is attached to the front side of the semiconductor wafer, so as to protect the devices formed on the front side of the wafer. The semiconductor wafer with the protective tape is held under suction through the protective tape on a chuck table included in the grinding apparatus, and the back side of the wafer held on the chuck table is ground by a grinding unit included in the grinding apparatus. 
     SUMMARY OF THE INVENTION 
     In the case of a MEMS (Micro Electro Mechanical Systems) wafer, a microscopic structure is formed in each device formed on the front side of the wafer. When the protective tape is attached to the front side of the MEMS wafer to protect the devices prior to grinding the back side of the wafer, there is a possibility that the microscopic structure in each device may be damaged in peeling the protective tape after the grinding step. Further, in the case of an imaging device wafer having a plurality of imaging devices such as CMOSs and CCDs formed on the front side, there is a possibility that when the protective tape is peeled from the front side of the wafer, a tack layer constituting the protective tape may be left on the front side of the wafer to cause device failure. 
     It is therefore an object of the present invention to provide a device wafer processing method which can reduce the possibility of damage to each device or device failure. 
     In accordance with an aspect of the present invention, there is provided a device wafer processing method of processing a device wafer including a device area where a plurality of devices are formed on the front side of the device wafer and a peripheral marginal area surrounding the device area, each device having an adhesion disliking region disliking the adhesion to an adhesive tape, the device wafer processing method including a protective tape attaching step of attaching an ultraviolet curable protective tape as the adhesive tape to the front side of the device wafer; an ultraviolet radiation applying step of applying ultraviolet radiation to a first area of the ultraviolet curable protective tape corresponding to at least the adhesion disliking region of the device wafer to thereby reduce the adhesive force in the first area in the condition where the ultraviolet radiation is not applied to a second area of the ultraviolet curable protective tape corresponding to at least the peripheral marginal area of the device wafer to thereby maintain the adhesive force in the second area, before or after performing the protective tape attaching step; and a grinding step of holding the device wafer through the ultraviolet curable protective tape and grinding the back side of the device wafer after performing the protective tape attaching step and the ultraviolet radiation applying step. 
     Preferably, the ultraviolet radiation is applied to a third area of the ultraviolet curable protective tape corresponding to the device area of the device wafer in the ultraviolet radiation applying step. 
     In the present invention, an ultraviolet curable tape having a tack layer to be cured by the irradiation with ultraviolet radiation is used as the protective tape for protecting the devices. Prior to performing the grinding step of grinding the back side of the device wafer, ultraviolet radiation is applied to the first area of the ultraviolet curable protective tape corresponding to at least the adhesion disliking region of the device wafer, which region dislikes the adhesion to the protective tape, thereby reducing the adhesive force in the first area of the protective tape. 
     Thus, the adhesive force of the protective tape in the first area corresponding to at least the adhesion disliking region of the device wafer has already been reduced in peeling the protective tape after the grinding step. Accordingly, it is possible to reduce the possibility that the microscopic structure in each device as the adhesion disliking region in the device area may be damaged or device failure may be caused in peeling the protective tape. 
     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. 1A  is a perspective view of a device wafer having a plurality of MEMS devices formed on the front side as viewed therefrom; 
         FIG. 1B  is an enlarged plan view of a part of the device wafer shown in  FIG. 1A ; 
         FIG. 2  is a perspective view showing a manner of preliminarily applying ultraviolet radiation to an area of an ultraviolet curable protective tape corresponding to a device area of the device wafer and next attaching the protective tape to the front side of the device wafer; 
         FIG. 3  is a perspective view showing a manner of attaching an ultraviolet curable protective tape to the front side of the device wafer; 
         FIG. 4  is a sectional view showing a manner of applying ultraviolet radiation to the area of the protective tape corresponding to the device area of the device wafer after performing the step shown in  FIG. 3 ; 
         FIG. 5  is a perspective view showing a grinding step of grinding the back side of the device wafer; and 
         FIG. 6  is an enlarged plan view of a part of the device wafer, showing another preferred embodiment of the ultraviolet radiation applying area. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will now be described in detail with reference to the drawings. Referring to  FIG. 1A , there is shown a perspective view of a device wafer  11  having a plurality of MEMS devices  15  formed on a front side  11   a .  FIG. 1B  is an enlarged plan view of a part of the device wafer  11  shown in  FIG. 1A . The device wafer  11  shown in  FIG. 1A  is a silicon wafer having a thickness of 700 μm, for example. A plurality of crossing division lines (streets)  13  are formed on the front side  11   a  of the device wafer  11  to thereby define a plurality of separate regions where the plural MEMS devices  15  are respectively formed. 
     The front side  11   a  of the device wafer  11  includes a device area  17  where the plural MEMS devices  15  are formed and a peripheral marginal area  19  surrounding the device area  17 . The outer circumference of the device wafer  11  is formed with a notch  21  as a mark for indicating the crystal orientation of the silicon wafer. 
     As shown in  FIG. 1B , each MEMS device  15  has a microscopic structure  23 . The microscopic structure  23  is a structure such that it is connected at one end  23   a  to the device wafer  11  in a cantilever fashion and most of the microscopic structure  23  floats over the device wafer  11 . When a protective tape is attached to the microscopic structure  23 , there is a possibility that the microscopic structure  23  may be damaged in peeling the protective tape. That is, the microscopic structure  23  is a region disliking the adhesion to the protective tape, and this region is therefore defined as “adhesion disliking region” in the description and claims. 
     Referring to  FIG. 2 , there is shown a perspective view for illustrating a first preferred embodiment of the protective tape attaching step and the ultraviolet radiation applying step in the present invention. Reference numeral  25  denotes an ultraviolet curable protective tape. The protective tape  25  shown in  FIG. 2  is originally in the form of a belt in this preferred embodiment. The protective tape  25  is composed of a base sheet  27  and a tack layer (adhesive layer)  29  formed on the base sheet  27 . The base sheet  27  is formed of polyolefin (PO) or polyvinyl chloride (PVC), for example. The tack layer  29  is formed of rubber adhesive or acrylic adhesive, for example. The tack layer  29  has a property such that it is cured by the irradiation with ultraviolet radiation to cause a reduction in adhesive force. 
     Reference numeral  31  denotes an area corresponding to the device wafer  11 , showing the outline of the device wafer  11 . In the first preferred embodiment shown in  FIG. 2 , ultraviolet radiation is first applied to an area  33  corresponding to the device area  17  of the device wafer  11  before attaching the protective tape  25  to the front side  11   a  of the device wafer  11 , thereby curing the tack layer  29  in the area  33  of the protective tape  25  to reduce the adhesive force in the area  33  (ultraviolet radiation applying step). 
     After performing the ultraviolet radiation applying step, the protective tape  25  is attached to the front side  11   a  of the device wafer  11  in an area  35  corresponding to the peripheral marginal area  19  of the device wafer  11 , and the protective tape  25  is cut along the outline  31  of the device wafer  11  (protective tape attaching step). Accordingly, the area  35  of the protective tape  25  is strongly attached to the peripheral marginal area  19  of the device wafer  11 , and the area  33  of the protective tape  25  is weakly attached to the device area  17  of the device wafer  11  by a small adhesive force because the tack layer  29  of the protective tape  25  has been cured by the ultraviolet radiation. 
     A second preferred embodiment of the protective tape attaching step and the ultraviolet radiation applying step will now be described with reference to  FIGS. 3 and 4 . As shown in  FIG. 3 , the ultraviolet curable protective tape  25  has substantially the same outline as that of the device wafer  11  in this preferred embodiment. This protective tape  25  is first attached to the front side  11   a  of the device wafer  11  (protective tape attaching step). After performing the protective tape attaching step, the ultraviolet radiation applying step is performed as shown in  FIG. 4 . That is, a mask  10  having an opening  10   a  corresponding to the device area  17  of the device wafer  11  is prepared, and the device wafer  11  is placed on the mask  10  in the condition where the device area  17  of the device wafer  11  is opposed through the protective tape  25  to the opening  10   a  of the mask  10 . In this condition, ultraviolet radiation is applied from an ultraviolet lamp  12  located below the mask  10  to the protective tape  25 , thereby curing the tack layer  29  of the protective tape  25  in the area  33  corresponding to the device area  17  of the device wafer  11  to reduce the adhesive force in the area  33  (ultraviolet radiation applying step). 
     The area  35  of the protective tape  25  corresponding to the peripheral marginal area  19  of the device wafer  11  is covered with the mask  10 . Accordingly, the ultraviolet radiation from the ultraviolet lamp  12  is not applied to the area  35  of the protective tape  25 , thereby preventing a reduction in adhesive force in this area  35 . After performing the protective tape attaching step according to the first preferred embodiment or performing the ultraviolet radiation applying step according to the second preferred embodiment as mentioned above, a grinding step is performed in such a manner that the device wafer  11  is held under suction through the protective tape  25  and a back side  11   b  of the device wafer  11  is ground. 
     This grinding step is performed by a grinding unit  14  (see  FIG. 5 ) of a grinding apparatus (not shown). As shown in  FIG. 5 , the grinding unit  14  includes a spindle  16  adapted to be rotationally driven, a wheel mount  18  fixed to the lower end of the spindle  16 , and a grinding wheel  20  detachably mounted on the lower surface of the wheel mount  18  by a plurality of screws  22 . The grinding wheel  20  is composed of an annular wheel base  24  and a plurality of abrasive members  26  fixed to the lower surface of the annular wheel base  24  so as to be annularly arranged at given intervals. The grinding apparatus further includes a chuck table  28  for holding the device wafer  11  under suction. As shown in  FIG. 5 , the device wafer  11  is held on the chuck table  28  under suction in the condition where the protective tape  25  is in contact with the chuck table  28  and the back side  11   b  of the device wafer  11  is exposed. 
     Thereafter, the chuck table  28  is rotated in the direction shown by an arrow a in  FIG. 5  at 300 rpm, for example, and the grinding wheel  20  is also rotated in the same direction as the rotational direction of the chuck table  28 , i.e., in the direction shown by an arrow b in  FIG. 5  at 6000 rpm, for example. Further, a grinding unit feeding mechanism (not shown) is operated to bring the abrasive members  26  of the grinding wheel  20  into contact with the back side  11   b  of the device wafer  11 . Thereafter, the grinding wheel  20  is fed downward by a predetermined amount at a predetermined feed speed, thereby grinding the back side  11   b  of the device wafer  11 . In grinding the back side  11   b  of the device wafer  11 , a contact type or noncontact type thickness gauge (not shown) is used to measure the thickness of the device wafer  11  until the thickness of the device wafer  11  is reduced to a desired thickness, e.g., 100 μm. 
     The protective tape  25  is firmly attached to the front side  11   a  of the device wafer  11  in only the area  35  corresponding to the peripheral marginal area  19  of the device wafer  11 . Accordingly, although the adhesive force in the area  33  of the protective tape  25  corresponding to the device area  17  of the device wafer  11  is reduced by the application of the ultraviolet radiation, there is no possibility that the protective tape  25  may be undesirably peeled from the front side  11   a  of the device wafer  11  by the application of a predetermined grinding load in the grinding step mentioned above. 
     After performing the grinding step, the protective tape  25  is forcibly peeled from the front side  11   a  of the device wafer  11 . The adhesive force in the area  33  of the protective tape  25  corresponding to the device area  17  of the device wafer  11  is reduced by applying the ultraviolet radiation to the area  33  to cure the tack layer  29  in this area  33 . Accordingly, the protective tape  25  can be peeled from the front side  11   a  of the device wafer  11  without causing damage to the microscopic structure (adhesion disliking region)  23  in each device  15 . 
     Another preferred embodiment of the ultraviolet radiation applying area will now be described with reference to  FIG. 6 . In this preferred embodiment, the ultraviolet radiation is applied to only the area of the protective tape  25  corresponding to the microscopic structure (adhesion disliking region)  23  in each device  15 , thereby curing the tack layer  29  in this area to reduce the adhesive force in this area. Accordingly, the ultraviolet radiation is not applied to the remaining area of the protective tape  25  except the area corresponding to the microscopic structure  23  in each device  15 , so that the adhesive force in the remaining area is not reduced. That is, the adhesive area where the protective tape  25  is firmly attached to the front side  11   a  of the device wafer  11  can be increased, so that the device wafer  11  can be fixed more firmly in the grinding step. 
     The ultraviolet radiation is applied to only the area of the protective tape  25  corresponding to the microscopic structure  23  in each device  15  in the following manner. First, the protective tape  25  is attached to the front side  11   a  of the device wafer  11 . Thereafter, the ultraviolet radiation is applied to the protective tape  25  by using a mask having openings respectively corresponding to the microscopic structures  23 . As a modification, prior to attaching the protective tape  25  to the front side  11   a  of the device wafer  11 , the ultraviolet radiation may be applied to the protective tape  25  by using a similar mask. Thereafter, the area of the protective tape  25  where the ultraviolet radiation has been applied is aligned to the microscopic structures  23 , and the protective tape  25  is then attached to the front side  11   a  of the device wafer  11 . 
     While the present invention is applied to the device wafer  11  having the MEMS devices  15  formed on the front side  11   a  in the above preferred embodiment, the present invention is not limited to the above preferred embodiment, but applicable to a device wafer having a plurality of imaging devices such as CMOSs and CCDs formed on the front side. 
     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.