Abstract:
A method of manufacturing a semiconductor device includes the steps of: preparing a bonding sheet having one or more holes that penetrate from a first surface to an opposite second surface thereof, and a semiconductor wafer having a semiconductor element; affixing the bonding sheet to a predetermined surface of the semiconductor wafer; and evacuating gas present between the bonding sheet and the semiconductor wafer via the one or more holes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing a semiconductor device and a method of manufacturing a bonding sheet. In particular, the present invention relates to a method of affixing a bonding sheet to a semiconductor wafer and a method of manufacturing a bonding sheet in connection with such affixing method. Moreover, the present invention relates to a semiconductor device having such bonding sheet. 
     2. Background Information 
     A method of cutting a semiconductor wafer into semiconductor device chips while the semiconductor wafer has a bonding sheet affixed thereto is known in the prior art (e.g. Japanese Laid-Open Patent Application No. 2000-340526 (hereinafter to be referred to as Patent Reference 1)).  FIG. 1A  and  FIG. 1B  are diagrams showing a conventional method of affixing a bonding sheet to a semiconductor wafer. 
     As shown in  FIG. 1A , in this conventional example, a bonding sheet  911  rolled up in a sheet supply spool  910  is used. First, a semiconductor wafer  940  having its back surface facing upward (i.e., the side opposite to the surface where circuit elements are formed) is placed on a wafer vacuum contact table  930  having a heater  931  built therein. Then, the bonding sheet  911  drawn out from the sheet supply spool  910  is pressed into the back surface of the semiconductor wafer  940  by a sheet affixing roller  950 . Here, the bonding sheet  911  should produce viscosity when heated. Therefore, in pressing the drawn out bonding sheet  911  onto the back surface of the semiconductor wafer  940 , the semiconductor wafer  940  is heated by the heater  931  in order to have the bonding sheet  911  affixed on the back surface of the semiconductor wafer  940 . An excess portion (hereinafter to be referred to as an excess sheet) of the bonding sheet  911  drawn out from the sheet supply spool  910  is taken up by a collecting spool  920  (hereinafter to be referred to as an excess sheet collecting spool). 
     The semiconductor wafer  940  having the bonding sheet  911  affixed thereto in the above described manner (q.v.  FIG. 1B ) is cut into chips of individual semiconductor devices using a dicing blade, for instance. After that, each chip of the semiconductor device is attached to a lead frame etc. using the bonding sheet  911  on the back surface thereof. 
     However, the conventional method as described above requires a number of expensive devices such as the sheet supply spool  910 , the excess sheet connection spool  920 , the wafer vacuum contact table  930  with the built-in heater  931  etc. Accordingly, equipment investment becomes costly, which makes it difficult to manufacture a semiconductor device at low cost. 
     In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved method of manufacturing a semiconductor device, an improved bonding sheet for use in a semiconductor device and an improved method of manufacturing a bonding sheet. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to resolve the above-described problems and to provide a bonding sheet which enables a semiconductor device to be manufactured at low cost and a method of manufacturing a semiconductor device at low cost. 
     In accordance with a first aspect of the present invention, a method of manufacturing a semiconductor device comprises the steps of: preparing a bonding sheet having one or more holes that penetrate from a first surface to an opposite second surface, and a semiconductor wafer having a semiconductor element; affixing the bonding sheet to a predetermined surface of the semiconductor wafer; and evacuating gas present between the bonding sheet and the semiconductor wafer via the holes. 
     In accordance with a second aspect of the present invention, a method of forming a bonding sheet comprises the steps of: preparing a bonding sheet; and punching holes into the bonding sheet using a mold having needles arranged in a two dimensioned array. 
     In accordance with a third aspect of the present invention, a bonding sheet which may be affixed to a semiconductor wafer comprises a base sheet having one ore more holes that penetrate from a first surface to an opposite second surface thereof. 
     In accordance with a fourth aspect of the present invention, the diameter of the holes in the bonding sheet according to the third aspect is greater than 0.5 mm and smaller than 1.0 mm. 
     In accordance with a fifth aspect of the present invention, in the bonding sheet according to the third aspect, the total area of the holes per unit area is less than 25 percent of the unit area. 
     In accordance with a sixth aspect of the present invention, the bonding sheet according to the third aspect has a base sheet that has thermal plasticity. 
     In accordance with a seventh aspect of the present invention, the bonding sheet according to the third aspect further comprises a thermoplastic film formed on the upper or lower surfaces of the base sheet, and the base sheet is made of polyimide or polytetrafluoroethylene. 
     In accordance with an eighth aspect of the present invention, the thermoplastic film of the bonding sheet according to the seventh aspect contains at least one compound selected from the group consisting of polyethylene, polystyrene, polypropylene, and polyvinyl chloride. 
     In accordance with a ninth aspect of the present invention, the bonding sheet according to the third aspect has a base sheet that has one or more liner bulges or liner trenches on either of the upper or lower surfaces thereof. 
     These and other objects, features, aspects, and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the attached drawings which form a part of this original disclosure: 
         FIG. 1A  and  FIG. 1B  are diagrams showing a conventional method of affixing a bonding sheet to a semiconductor wafer; 
         FIG. 2A  and  FIG. 2B  are diagrams showing a structure of a bonding sheet according to a first embodiment of the present invention; 
         FIG. 3A  to  FIG. 3C  are overhead views showing manufacturing processes of a semiconductor device according to the first embodiment of the present invention; 
         FIG. 4A  to  FIG. 4C  are sectional views showing manufacturing processes of the semiconductor device according to the first embodiment of the present invention; 
         FIG. 5A  to  FIG. 5C  are sectional views showing manufacturing processes of a semiconductor device according to the second embodiment of the present invention; 
         FIG. 6A  is a diagram showing a structure of a modified bonding sheet according to the present invention; and 
         FIG. 6B  is a sectional view of the modified bonding sheet according to the present invention corresponding to a cross section taken along the line II-II′ in  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Selected embodiments of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     First Embodiment 
     First, a first embodiment of the present invention will be described in detail with reference to the drawings.  FIG. 2A  and  FIG. 2B  are diagrams showing the structure of a bonding sheet  11  according to the first embodiment of the present invention. 
     As shown in  FIG. 2A , the bonding sheet  11  is supplied from a rolled sheet  10 . In using the bonding sheet  11 , as shown in  FIG. 2B , the bonding sheet  11  is cut out from the rolled sheet  10  as much as needed. 
     The bonding sheet  11 , for instance, is 1 mm (millimeter) thick, and a width thereof should preferably be wider than the diameter of a semiconductor wafer where multiple semiconductor elements are formed. Therefore, if the diameter of the semiconductor wafer is 6 inches, for instance, the width of the bonding sheet  11  should be 8 inches, for instance. 
     The bonding sheet  11  to be used here should be a thermoplastic sheet which softens by being heated. The thermoplastic bonding sheet  11  can be formed using a synthetic resin such as polyethylene, polystyrene, polypropylene, polyvinyl chloride, and the like, for instance. However, the material of the bonding sheet  11  is not limited to the ones mentioned above, and various kinds of commonly used materials may be used. In this particular embodiment, the bonding sheet  11  has a base made of a polyimide film or a polytetrafluoroethylene (PTFE) film with a thickness of about 50 μm (micrometers) or so, and a thermoplastic resin is applied to each side of this base sheet to a thickness of about 25 μm or so. In the structure of the bonding sheet  11  as described above, it is also possible to add some filler or the like to the base sheet composed of a polyimide film or a polytetrafluoroethylene film. By such arrangement, it is possible to improve the stiffness of the base sheet. 
     Furthermore, the boding sheet  11  has at least one hole  12  (preferably, a plurality of holes  12 ) which penetrates through the front and back sides of the bonding sheet  11 . The hole  12  is 0.5 mm (millimeters) in diameter (φ), for instance. However, the diameter of the hole  12  is not limited thereto. As long as the hole  12  has a diameter that is suitable to enable a semiconductor wafer  40  to be affixed to a wafer vacuum contact table  30  by exhausting gas from exhaust holes  32  in the wafer vacuum contact table  30 , and to ensure the strength needed in the bonding sheet  11 , the hole  12  may be varied in any possible way. For instance, a diameter (φ) of the hole  12  which fulfills such conditions would be within a range of 0.5 mm to 1.0 mm. 
     Furthermore, provided that the diameter (φ) of the hole  12  is 0.5 mm, for instance, the distance between the centers of two adjacent holes  12  (hereinafter to be referred to as the pitch P) may be 1.0 mm, for instance. However, the pitch P is not limited to this figure. As long as the pitch P is suitable to enable gas between the bonding sheet  11  and the semiconductor wafer  40  to be discharged evenly, and enable each individual semiconductor device cut out from the semiconductor wafer  40  to have enough area of the affixed boding sheet  11 , the figure for the pitch P may be varied. Normally, in order to secure sufficient affixing strength, an area of bonding sheet affixed to a semiconductor device should preferably be more than about 75% of the chip area of the semiconductor device. 
     Considering the above factors, when arranging the holes  12  in the bonding sheet  11  regularly (e.g., at predetermined intervals) in a two-dimensional array, the upper limit on the number of holes  12  to be formed per unit area (i.e. N) should fall within the range shown in Formula  1  below. Likewise, when arranging the holes  12  in the bonding sheet  11  regularly in a two-dimensional array, the pitch P can be calculated by Formula  2  as shown below using the number of holes  12  (i.e., N). In the following formulas, ‘S’ indicates the unit area and ‘Sa’ indicates the area per single hole  12 . Here, a substantial bonding area required for the bonding sheet  11  is set to be equal to or greater than 75% of a chip area of a diced semiconductor device chip. That is, the total area of the holes  12  formed in one unit area is set to be equal to or less than 25% of the chip area of the diced semiconductor device chip. 
     
       
         
           
             
               
                 
                   
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     The bonding sheet  11  can be formed by punching holes ( 12 ) into a bonding sheet ( 11 ) using a pressing machine in which a metal mold, such as a jig, having a large number of needles arranged thereon, is attached thereto. 
     Now, a method for manufacturing a semiconductor device using the bonding sheet  11  according to the first embodiment of the present invention will be described with reference to  FIG. 2B  and  FIG. 3A  to  FIG. 4C .  FIG. 4A  to  FIG. 4C  are drawn based on a section taken along a line I-I′ shown in  FIG. 3C . 
     In this manufacturing method, first, as shown in  FIG. 2B , a rather large portion of the bonding sheet  11  needed (e.g. 8 inches square) is cut out from the rolled sheet  10 . 
     Next, as shown in  FIG. 3A , the cut out bonding sheet  11  is placed on the wafer vacuum contact table  30 . As shown in  FIG. 3A , the loading surface of the wafer vacuum contact table  30  has a large number of exhaust holes  32  arranged in an array. As shown in  FIG. 3B , some of the exhaust holes  32  provided in the wafer vacuum contact table  30  overlap with the holes  12  in the bonding sheet  11 , and the other exhaust holes  32  overlap parts of the boding sheet  11  where the holes  12  are not formed. These exhaust holes  32  are connected with an evacuation device (not shown), e.g. a vacuum pump, for instance. That is, the wafer vacuum contact table  30  is formed so that it can evacuate gas from the exhaust holes  32 . 
     As shown in  FIG. 3C , the semiconductor wafer  40  is arranged on the wafer vacuum contact table  30  where the bonding sheet  11  is placed. By this arrangement, a predetermined surface of the semiconductor wafer  40  (e.g. the surface of the semiconductor wafer  40  where semiconductor elements are not formed) is placed in contact with the surface of the bonding sheet  11  where the holes  12  are formed. 
     As shown in  FIG. 4A , gas is evacuated from the exhaust holes  32  provided in the wafer vacuum contact table  30 , using the evacuation device (not shown in the figure). In other words, the interior of the wafer vacuum contact table  30  is drawn into a vacuum. By this arrangement, gas between the wafer vacuum contact table  30  and the bonding sheet  11  is evacuated from the exhaust holes  32  that are not overlapped with the holes  12  of the bonding sheet  11 , by which the bonding sheet  11  is affixed to the wafer vacuum contact table  30 . Furthermore, as gas between the wafer vacuum contact table  30  and the semiconductor wafer  40  is evacuated from the exhaust holes  32  which overlap with the holes  12  in the bonding sheet  11 , the semiconductor wafer  40  is aspirated toward the wafer vacuum contact table  30 , and at the same time, as gas between the bonding sheet  11  and the semiconductor wafer  40  is evacuated from the same exhaust holes  32 , the semiconductor wafer  40  coheres to the bonding sheet  11 . 
     At this time, as shown in  FIG. 4A , the bonding sheet  11  placed on the wafer vacuum contact table  30  is heated by having a heater  31  heat the wafer vacuum contact table  30 . By this arrangement, the bonding sheet  11  softens. As described above, because the semiconductor wafer  40  is cohered to the bonding sheet  11  while it is being aspirated toward the wafer vacuum contact table  30 , as the bonding sheet  11  softens, the bonding sheet  11  and the semiconductor wafer  40  are made to cohere with each other due to pressure generated between the bonding sheet  11  and the semiconductor wafer  40  by the aspiration. 
     After that, the wafer vacuum contact table  30 , the bonding sheet  11  placed on the wafer vacuum contact table  30  and the semiconductor wafer  40  are cooled to room temperature, for instance (here, it is appropriate as long as the temperature is equal to or lower than a temperature at which the bonding sheet  11  hardens again). Then, as shown in  FIG. 4B , the bonding sheet  11  and the semiconductor wafer  40  are detached from the wafer vacuum contact table  30 . 
     Next, the semiconductor wafer  40  having the bonding sheet  11  affixed thereto through the above-described processes is fixed on a predetermined dicing stage and cut into pieces of individual semiconductor device (chips) using a dicing blade  100  etc., for instance, as shown in  FIG. 4C . By this arrangement, chips of individual semiconductor devices can be manufactured. 
     As described above, according to the first embodiment of the present invention, the semiconductor wafer  40  is aspirated by the holes  12  arranged in the bonding sheet  11 , and thereby, it is possible to have the same kind of effects as when applying pressure on the semiconductor wafer  40  to push it toward the bonding sheet  11 . In other words, according to this embodiment, it is possible to acquire enough pressure to have the bonding sheet  11  and the semiconductor wafer  40  stick to each other by aspiration. 
     In this embodiment, since the structure adopted for enabling aspiration can use a conventional structure for aspirating a semiconductor wafer to a wafer vacuum contact table, it is not necessary to prepare additional devices for this purpose. In addition, in the structure of this embodiment, it is not necessary to have a structure for drawing out the bonding sheet (e.g. the sheet supply spool  910  shown in  FIG. 1 ), a structure for pressing the bonding sheet into the semiconductor wafer (e.g. the sheet affixing roller  950  shown in  FIG. 1 ), a structure for collecting an excess bonding sheet (e.g. the excess sheet collecting spool  920 ), or the like, and therefore, it is possible to simplify the facilities. As a result, according to the present invention, it is possible to reduce the initial cost and running cost for the manufacturing facilities, which makes it possible to manufacture the semiconductor device at low cost. 
     Furthermore, since the bonding sheet  11  according to this embodiment has a structure in which a commonly used bonding sheet has multiple holes  12 , the bonding sheet  11  according to this embodiment can be manufactured based on the structure of a conventional bonding sheet through simple manufacturing processes at low cost. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described in detail with reference to the drawings. In the following, the structures that are the same as the first embodiment will have the same reference numbers used therewith, and redundant explanations of those structural elements will be omitted. 
     In this embodiment, the same bonding sheet  11  as described in the first embodiment of the present invention will be used, and another method of manufacturing a semiconductor device will be described. 
       FIG. 5A  to  FIG. 5C  are sectional views showing manufacturing processes of the semiconductor device according to the second embodiment of the present invention.  FIG. 5A  to  FIG. 5C  are drawn based on a section taken along a line I-I′ shown in  FIG. 3C  as with  FIG. 4A  to  FIG. 4C . 
     In this manufacturing method, first, as shown in  FIG. 5A , the semiconductor wafer  40  is placed on a heat-resistant board  51 , and is covered with the bonding sheet  11  cut out from the rolled sheet  10 . In this process, the bonding sheet  11  is cut out in a rather larger portion (e.g., 8 inches square) from the rolled sheet  10 , and a predetermined surface of the semiconductor wafer  40  (e.g. the surface of the semiconductor wafer  40  where semiconductor elements are formed) is placed in contact with the heat-resisting board  51 . By this arrangement, the opposite surface of the semiconductor wafer  40  (e.g. the surface of the semiconductor wafer  40  where semiconductor elements are not formed) is placed in contact with the bonding sheet  11 . In addition, instead of the heat-resistant board  51 , it is also possible to use a heat-resisting sheet. 
     Next, the semiconductor wafer  40  sandwiched between the heat-resisting board  51  and the bonding sheet  11  is wrapped with a material having air permeability and flexibility, e.g., a sponge  52 . Then, as shown in  FIG. 5B , the semiconductor wafer  40  wrapped with the sponge  52 , for instance, is set inside a vacuum defoaming chamber  53 , after which gas is evacuated from exhaust holes  32  provided in the wafer vacuum contact table  30 , using an evacuation device (not shown). In other words, the interior of the wafer vacuum contact table  30  is drawn into a vacuum. By this arrangement, as gas between the bonding sheet  11  and the semiconductor wafer  40  is evacuated from the holes  12 , the semiconductor wafer  40  coheres to the bonding sheet  11 . By having a structure in which the semiconductor wafer  40  sandwiched between the heat-resistant board  51  and bonding sheet  11  is wrapped in the sponge  52 , etc., as mentioned above, the bonding sheet  11  is pressed toward the semiconductor wafer  40  uniformly. Accordingly, it becomes easier to disperse the air bubbles existing between the bonding sheet  11  and the semiconductor wafer  40 , and thereby, it becomes possible to improve adhesiveness between the bonding sheet  11  and the semiconductor wafer  40  after evacuating the gas between them. 
     By these processes, the semiconductor wafer  40  is wrapped so that it is laminated by the heat-resisting board  51  and the bonding sheet  11 . 
     Next, the wrapped semiconductor wafer  40  is taken out from the vacuum defoaming chamber  53  and the sponge  52  wrapping the semiconductor wafer  40  is peeled off. Then, as shown in  FIG. 5C , the semiconductor wafer  40  having the bonding sheet  11  affixed thereto is set inside an oven  54  and heated. The bonding sheet  11  affixed to the semiconductor wafer  40  softens and coheres with the semiconductor wafer  40 . 
     After that, the semiconductor wafer  40  sandwiched between the bonding sheet  11  and the heat-resisting board  51  is taken out from the oven  54 , and the bonding sheet  11  and the semiconductor wafer  40  are cooled to room temperature, for instance (here, it is appropriate as long as the temperature is equal to or lower than a temperature at which the bonding sheet  11  hardens again). Then, the heat-resisting board  51  is detached from the semiconductor wafer  40 . 
     Next, the semiconductor wafer  40  having the bonding sheet  11  affixed thereto through the above-described processes is fixed on a predetermined dicing stage and cut into pieces of individual semiconductor devices (chips) using a dicing blade  100  etc., for instance, as described above with reference to  FIG. 4C . By this arrangement, chips of individual semiconductor devices can be manufactured. 
     As described above, according to the second embodiment of the present invention, the semiconductor wafer  40  is aspirated by means of the holes  12  arranged in the bonding sheet  11 , and thereby, it is possible to have the same kind of effects as when applying pressure on the semiconductor wafer  40  to push it toward the bonding sheet  11 . Furthermore, in making the bonding sheet  11  and the semiconductor wafer  40  stick together, since the bonding sheet  11  is uniformly pushed toward the semiconductor wafer  40  by a material having air permeability and flexibility such as the sponge  52 , it is possible to make the bonding sheet  11  and the semiconductor wafer  40  stick to each other without the bonding sheet  11  being wrinkled. Accordingly, it is possible to improve adhesiveness between the bonding sheet  11  and the semiconductor wafer  40 . 
     In this embodiment, since the bonding sheet  11  and the semiconductor wafer  40  are made to adhere to each other by being heated after the bonding sheet  11  is cohered to the semiconductor wafer  40 , it is not necessary to have a structure for drawing out the bonding sheet (e.g. the sheet supply spool  910  shown in  FIG. 1 ), a structure for pressing the boding sheet into the semiconductor wafer (e.g. the sheet affixing roller  950  shown in  FIG. 1 ), a structure for collecting an excess bonding sheet (e.g. the excess sheet collecting spool  920 ), or the like, and therefore, it is possible to simplify the manufacturing facilities. As a result, according to the present invention, it is possible to reduce the initial cost and running cost for the manufacturing facilities, which makes it possible to manufacture the semiconductor device at low cost. 
     Furthermore, since the bonding sheet  11  according to this embodiment has a structure in which a commonly used bonding sheet has multiple holes  12 , as in the case of the first embodiment, the bonding sheet  11  according to this embodiment can be manufactured based on the structure of a conventional bonding sheet through simple manufacturing processes and at low cost. 
     Although cases where a thermoplastic sheet is used as the bonding sheet  11  have been referred to in the above descriptions of the first and second embodiments, the present invention is not limited to this. For instance, instead of using a thermoplastic sheet as the bonding sheet  11 , it is possible to use a bonding sheet having a structure in which an adhesive material such as a resin is applied to at least on one surface thereof. In this case, the processes of softening the bonding sheet  11  by heating can be omitted. 
     Moreover, although a case in which some filler etc. is added to the base sheet, i.e., the polyimide film or the polytetrafluoroethylene film, in order to improve stiffness of the base sheet of the bonding sheet  11  has been described in the above descriptions of the first and second embodiments, the present invention is not limited to this. For instance, as shown in  FIG. 6A  and  FIG. 6B , it is possible to improve the stiffness of the base sheet of the bonding sheet  11  by forming liner bulges  22  (or liner trenches) on least on one surface of the base sheet, which is the polyimide film or the polytetrafluoroethylene film, with the liner bulges  22  (or liner trenches) crossing in a reticular pattern.  FIG. 6A  is a diagram showing a structure of such modified bonding sheet  11  according to the present invention, and  FIG. 6B  is a sectional view of the modified bonding sheet  11  according to the present invention corresponding to the cross section taken along the line II-II′ in  FIG. 6A . However, the shapes of bulges (or trenches) formed on the surface of the base sheet are not limited to the ones mentioned above, and any shape can be used so long as the stiffness of the base sheet of the bonding sheet  11  can be improved. 
     While the preferred embodiments of the invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or the scope of the following claims. 
     This application claims priority to Japanese Patent Application No. 2005-143629. The entire disclosures of Japanese Patent Application No. 2005-143629 is hereby incorporated herein by reference. 
     While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. Thus, the scope of the invention is not limited to the disclosed embodiments. 
     The term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function. 
     Moreover, terms that are expressed as “means-plus function” in the claims should include any structure that can be utilized to carry out the function of that part of the present invention. 
     The terms of degree such as “substantially,” “about,” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies.