Abstract:
In a semiconductor device, semiconductor chips are mounted on two surfaces of a die pad. A lower one of the semiconductor chips has a portion projecting outward from an upper one of the semiconductor chips. The semiconductor chips are connected to integrally molded external connection leads trough wiring members. The lower semiconductor chipping the outwardly projecting portion has, on its surface on the same side as an upper surface of the upper semiconductor chip, pads to be connected to the external connection leads. A semiconductor device lead frame and a method of manufacturing the same are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a chip stack type semiconductor device in which a plurality of semiconductor chips are mounted on a lead frame by the high-degree integration technique, and a semiconductor device lead frame and a method of manufacturing the same. 
     For example, a chip stack type semiconductor device of this type is used when logic cells and memory cells are to be mounted to form a hybrid circuit, when a memory capacity is to be increased, and when a plurality of semiconductors having different manufacturing processes and materials are to be mounted. 
     A structure conventionally known as a chip stack type semiconductor device of this type will be described with reference to FIG.  8 . In this conventional semiconductor device  1 , first and second semiconductor chips  3  and  4  are respectively adhered to the upper and lower surfaces of a die pad  2  of a lead frame L/F with conductive adhesive portions  5   a  and  5   b  of a silver paste or the like. Bonding wires  7  and  8  for respectively connecting the chips  3  and  4  to inner leads  6  of the lead frame L/F are connected to the upper and lower surfaces, respectively, of the inner leads  6 . A resin-encapsulated package  9  encapsulates the chips  3  and  4  stacked on and adhered to the die pad  2 , the inner leads  6  of the lead frame, and the bonding wires  7  and  8 . 
     This conventional semiconductor device is manufactured in accordance with the following manufacturing method. 
     The conductive adhesive portion  5   a  of a silver paste or the like is formed on the surface of the die pad  2  of the lead frame L/F, and the first chip  3  is mounted on it. The lead frame L/F mounted with the first chip  3  is turned back. The conductive adhesive portion  5   b  of a silver paste or the like is formed on the lower surface of the die pad  2 , and the second chip  4  is mounted on it. The resultant structure is baked. 
     The second chip  4  and the lower surfaces of the inner leads  6  are connected to each other by bonding with the bonding wires  8 . After that, the lead frame L/F is turned over. The first chip  3  and the upper surfaces of the inner leads  6  are connected to each other by bonding with the bonding wires  7 . After that, resin encapsulation and following operation are performed. An explanation of resin encapsulation and the following operation will be omitted. 
     FIG. 9 shows a case wherein a semiconductor device  1  uses a TAB tape  10 . In FIG. 9, the TAB tape  10  is adhered to a portion of the surface of a die pad  2 , extending laterally from a first chip  3 , through an adhesive  11 . 
     Metal wires  12  on the surface of the TAB tape  10  and the first chip  3  are connected to each other by bonding with bonding wires  7 . The metal wires  12  of the TAB tape  10  are also connected to inner leads  6  by bonding with bonding wires  13 . 
     Even in the semiconductor device  1  having this structure, the first chip  3  and a second chip  4  on the upper and lower surfaces, respectively, of the die pad  2  are connected to the inner leads  6  from both the upper and lower sides by turning a lead frame L/F over during the manufacture. 
     In the semiconductor device  1  having the conventional structure described above, as shown in FIGS. 8 and 9, to connect the first chip  3  by bonding, the second chip  4  must be connected by bonding, then the lead frame L/F must be turned over, and after that, the first chip  3  must be connected by bonding. As the bonding step must be performed twice during the manufacturing process, the number of manufacturing steps and accordingly the cost are increased. 
     After bonding connection of the second chip  4  is completed, the lead frame L/F is turned over to connect the first chip  3  by bonding. Therefore, the bonding wires  7 ,  8 , and  13  tend to deform easily during the manufacture. The semiconductor device  1  thus must be handled carefully, and the yield in the manufacture decreases. As described above, the manufacture requires turning the lead frame over and performing connection by bonding on the two surfaces of the lead frame. A conventionally used manufacturing apparatus cannot be used directly, but a special apparatus is needed. 
     In the structure shown in FIG. 9, the TAB tape  10  is used as an interconnection in a resin-encapsulated package  9  for connecting the first and second chips  3  and  4 . This increases the cost of the lead frame L/F. 
     For example, Japanese Patent Laid-Open No. 3-116860 discloses a chip stack type semiconductor device. In this semiconductor device, insulating adhesive tapes are adhered to both the upper and lower surfaces of a lead frame, and first and second chips having different sizes are placed on the upper and lower surfaces of the lead frame. The chips and the inner leads of the lead frame are connected to each other by wire bonding. 
     In the semiconductor device disclosed in this reference, when the respective chips are mounted on the upper and lower surfaces of the lead frame through insulating adhesive films, two expensive insulating adhesive films are required to increase the material cost. In this conventional structure, when the chips are mounted on the upper and lower surfaces of the lead frame through the insulating adhesive films, the thickness of the package is difficult to decrease in the presence of the two insulating adhesive films. 
     Japanese Patent Laid-Open No. 7-38050 discloses a semiconductor device manufactured by using an LOC (Lead On Chip)-structure lead frame having isolated inner leads for internal interconnection. 
     As shown in this reference, it is technically difficult to cut outwardly extending inner leads and insulating tapes simultaneously. In the lead frame used in this conventional structure, the insulating tapes to which the isolated inner leads for internal interconnection are adhered are supported by the outwardly extending inner leads. Therefore, design of the inner leads for internal interconnection is limited due to the space. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above situation, and has as its object to provide a semiconductor device in which, when semiconductor chips having different sizes are to be mounted on the die pad of a lead frame and the respective chips are to be connected to the inner leads of the lead frame by using bonding wires, the number of manufacturing steps is reduced, the manufacturing operability is improved, and the manufacturing cost is reduced, and a semiconductor device lead frame and a method of manufacturing the same. 
     In order to achieve the above object, according to the present invention, there is provided a semiconductor device in which semiconductor chips are mounted on two surfaces of a die pad, a lower one of the semiconductor chips having a portion projecting outward from an upper one of the semiconductor chips, and the semiconductor chips being connected to integrally molded external connection leads through wiring members, wherein the lower semiconductor chip having the outwardly projecting portion has, on a surface thereof on the same side as an upper surface of the upper semiconductor chip, pads to be connected to the external connection leads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a semiconductor device and a semiconductor device lead frame and a method of manufacturing the same according to the first embodiment of the present invention; 
     FIG. 2 is a plan view of the semiconductor device and the semiconductor device lead frame and the method of manufacturing the same according to the first embodiment of the present invention; 
     FIG. 3 is a sectional view of a semiconductor device and a semiconductor device lead frame and a method of manufacturing the same according to the second embodiment of the present invention; 
     FIG. 4 is a plan view of the semiconductor device and the semiconductor device lead frame and the method of manufacturing the same according to the second embodiment of the present invention; 
     FIG. 5 is a plan view of a semiconductor device and a semiconductor device lead frame and a method of manufacturing the same according to the third embodiment of the present invention; 
     FIGS. 6A to  6 C are plan views of the semiconductor device and the semiconductor device lead frame and the method of manufacturing the same according to the third embodiment of the present invention; 
     FIG. 7 is a plan view of a semiconductor device and a semiconductor device lead frame and a method of manufacturing the same according to the fourth embodiment of the present invention; 
     FIG. 8 is a sectional view of a conventional semiconductor device; and 
     FIG. 9 is a sectional view showing another example of a conventional semiconductor device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show a semiconductor device and a semiconductor device lead frame and a method of manufacturing the same according to the first embodiment of the present invention. 
     Referring to FIGS. 1 and 2, reference numeral  20  denotes a chip stack type semiconductor device. In the semiconductor device  20 , a first semiconductor chip  24  is fixed to the upper surface of a die pad  22  of a lead frame  21  by adhesion through a conductive adhesive  23  such as a silver paste, and a second semiconductor chip  26  is fixed to the lower surface of the die pad  22  by adhesion through an adhesive tape  25  such as an LOC tape. 
     The first and second chips  24  and  26  have different chip sizes, and the second chip  26  is larger than the first chip  24 . Pads  24   a  and  26   a  are formed on the upper surface of the first chip  24  and the upper surface of the second chip  26  (a surface on a side where the second chip  26  is fixed to the die pad  22  by adhesion and which extends laterally from the first chip  24 ), respectively, to connect the first and second chips  24  and  26  to inner leads  27  of the lead frame  21  through bonding wires  28  and  29 . 
     Although not shown, the first and second chips  24  and  26  may be electrically connected to each other by connecting the pads  24   a  on the first chip  24  and the pads  26   a  on the second chip  26  to each other by bonding directly through bonding wires. 
     In FIG. 1, a resin-encapsulated package  20   a  encapsulates the chips  24  and  26 , the inner leads  27 , and the like described above, and a hanging pin  30  supports the die pad  22 . 
     A method of manufacturing the semiconductor device  20  having the above arrangement will be described. 
     The lead frame  21 , in which the adhesive tape  25  such as the LOC tape is adhered to the lower surface of the die pad  22 , is prepared. The second chip  26  is thermally bonded to the lower surface of the die pad  22  and fixed to the die pad  22  by adhesion. 
     The conductive adhesive  23  such as a silver paste is applied to the upper surface of the die pad  22 . The first chip  24  is mounted on and adhered to the conductive adhesive  23 . The resultant structure is baked. 
     After that, the upper surface of the first chip  24  and the upper surface of the second chip  26  are connected to the upper surfaces of the inner leads  27  by bonding with the bonding wires  28  and  29 . The entire structure is subjected to resin encapsulation and following operation. Resin encapsulation and the following operation are performed in the same manner as in the conventional manufacturing method, and a description thereof will be omitted. 
     According to the semiconductor device  20  having this arrangement, the first and second chips  24  and  26  can be fixed to the die pad  22  and the pads of the chips  24  and  26  are connected to the inner leads  27  by bonding with the bonding wires  28  and  29  from the upper side of the device simultaneously, or without turning the device over. Accordingly, unlike in the conventional method, a complicated, cumbersome operation of turning the lead frame over twice during the manufacture in order to fix the first and second chips to the die pad by adhesion and to connect the chips and the inner leads by bonding is not necessary. 
     FIGS. 3 and 4 show the second embodiment of the present invention. In FIGS. 3 and 4, portions identical or corresponding to their counterparts in FIGS. 1 and 2 described above are denoted by the same reference numerals as in FIGS. 1 and 2, and a detailed description thereof will be omitted. 
     In the second embodiment, LOC lead portions  27   a  of inner leads  27  are adhered to the upper surface portion of a second chip  26 , extending laterally from a first chip  24 , through adhesive tapes  31  such as LOC tapes, thus forming an interconnection in a package  20   a.    
     Referring to FIGS. 3 and 4, bonding wires  32  connect the first chip  24  and the LOC lead portions  27   a  to each other, and bonding wires  33  connect the LOC lead portions  27   a  and the inner leads  27  to each other. In the second embodiment, the die pad  22  is slightly larger than the first chip  24 . 
     The first and second chips  24  and  26  may be electrically connected to each other by connecting pads  24   a  on the first chip  24  described above and pads  26   a  on the chip  26  directly by bonding. 
     A method of manufacturing the semiconductor device  20  described above according to the second embodiment of the present invention will be described. 
     A lead frame is prepared in the following manner. An adhesive tape  25  and the adhesive tapes  31  such as LOC tapes are adhered to the lower surface of a die pad  22  and the lower surfaces of the inner leads (LOC lead portions  27   a ), that form the LOC structure portion. The second chip  26  is thermally bonded to the lower surfaces of the die pad  22  and LOC lead portions  27   a , thereby fixing it. 
     A conductive adhesive  23  such as a silver paste is applied to the upper surface of the die pad  22 . The first chip  24  is adhered to the conductive adhesive  23 . The resultant structure is baked. 
     After that, the first and second chips  24  and  26 , the inner leads  27 , and the LOC lead portions  27   a  are connected to each other by bonding, and operation following bonding is performed. A description of the operation following bonding will be omitted. 
     According to a semiconductor device  20  having this structure and the method of manufacturing the same, since the upper surface of the second chip  26  has an LOC structure, the interconnection in the package  20   a  to connect the first and second chips  24  and  26  to each other can be formed without using a TAB tape. 
     FIG. 5 shows a semiconductor device according to the third embodiment of the present invention. In FIG. 5, portions identical or corresponding to their counterparts in FIG. 4 described above are denoted by the same reference numerals as in FIG. 4, and a detailed description thereof will be omitted. 
     In the third embodiment, independent inner leads  35  for internal interconnection of the package are adhered to the upper surface of a second chip  26  through adhesive tapes  31  such as LOC tapes, to form an interconnection in a package  20   a.    
     The first and second chips  24  and  26  may be electrically connected to each other by connecting pads  24   a  on a first chip  24  and pads  26   a  on the second chip  26  directly by bonding. 
     According to the third embodiment, the interconnection in the package  20   a  completely independent of the external leads can be formed. As compared to the structure of the second embodiment described above, inner leads  27  need not be extended onto the second chip  26  in order to form an LOC portion. There are no pads which cannot be connected by bonding due to the fact that the inner leads  27  extend to pass above the upper surface portions of the pads  26   a  of the second chip  26 . Therefore, the degree of freedom in design can be widened. 
     A method of manufacturing the semiconductor device  20  described above according to the third embodiment will be described above. 
     A lead frame is prepared in the following manner. An adhesive tape  25  such as an LOC tape is adhered to the lower surface of the die pad  22 , and adhesive tapes  25  such as LOC tapes, adhered with the independent inner leads  35  for interconnection in the package at the peripheral portion of a die pad  22 , are held with hanging pins  30 . 
     The second chip  26  is thermally bonded to the lower surface of the die pad  22 . A conductive adhesive  23  such as a silver paste is applied to the upper surface of the die pad  22 , and the first chip  24  is adhered to the conductive adhesive  23 . The resultant structure is baked. 
     Operation following bonding is performed. The operation following bonding is the same as that described above which is conventionally, widely known, and accordingly a description thereof will be omitted. 
     A method of manufacturing a semiconductor device lead frame used in the third embodiment will be described with reference to FIGS. 6A,  6 B, and  6 C. 
     First, operation is performed until interior plating (silver plating) and cutting the distal ends of leads in accordance with the same steps as those performed for forming an ordinary lead frame. 
     At this time point, the inner leads  35  for interconnection in the package are connected to the die pad  22 , and are formed as tab portions  35   a  extending from the die pad  22 , as shown in FIG.  6 A. 
     The adhesive tapes  25  and  31  such as LOC tapes are adhered to the lower surface of the die pad  22  and the lower surfaces of the tab portions  35   a  extending from the die pad  22 , as shown in FIG.  6 B. 
     At this time, the distance between the adhesive tape  25  adhered to the lower surface of the die pad  22  and the adhesive tapes  31  adhered to the lower surfaces of the tab portions  35   a  extending from the die pad  22  is preferably set to at least 1 mm or more considering the amounts of the tab portions  35   a  left after cutting. 
     The peripheral portion of the die pad  22  excluding the hanging pins  30  is cut to separate the tab portions  35   a  and die pad  22  from each other, thus forming the independent inner leads  35  for interconnection in the package. In FIG. 6C, reference numeral  36  denotes a portion where the tab portions  35   a  and die pad  22  are separated from each other. 
     Since the adhesive tapes  31  that adhere the inner leads  35  for interconnection in the package also adhere the hanging pins  30 , the tapes  31  are supported by the hanging pins  30 . Thus, the respective inner leads  35  for interconnection in the package become independent of each other, and can maintain the shape as a lead frame  40  even if they do not extend outside the package. 
     It is confirmed from experiments that, if the semiconductor device  20  is one having a general size, the width of a portion of each hanging pin that overlaps the second chip  26  is preferably at least 0.6 mm or more. 
     FIG. 7 shows a semiconductor device according to the fourth embodiment of the present invention, which will be described hereinafter. 
     A semiconductor device  20  according to the fourth embodiment has a package  20   a  having a following structure (LOC structure). That is, a second chip  26  is adhered to the lower surface of a lead frame through adhesive tapes  31  such as LOC tapes adhered to the lower surfaces of inner leads  27 . In this package  20   a , part (an inner end portion) of each inner lead  27  is formed to have a square or rectangular shape. A first chip  24  is adhered to the upper surfaces of these square or rectangular portions (inner leads  27 ) through a conductive adhesive  23  such as a silver paste. 
     Referring to FIG. 7, bonding wires  37  and  38  respectively connect the inner ends of the inner leads  27  and pads  26   a  of the second chip  26  to each other, and pads  24   a  of the first chip  24  and the pads  26   a  of the second chip  26  to each other. 
     A method of manufacturing the semiconductor device  20  according to the fourth embodiment will be described. 
     A lead frame is prepared in the following manner. Inner leads  27  are formed to partly have square or rectangular shapes, and an adhesive tape  25  and the adhesive tapes  31  such as LOC tapes are adhered to the lower surfaces of the square or rectangular portions partly formed at least on the inner leads  27 , and to the lower surfaces of the bonding portions of the inner leads  27 . 
     The second chip  26  is thermally bonded to the lower surface of the lead frame through the adhesive tapes  25  and  31  such as the LOC tapes. The conductive adhesive  23  such as a silver paste is applied to the upper surfaces of the square or rectangular portions formed on the inner leads  27 , and the first chip  24  is adhered. The resultant structure is baked. 
     After that, operation following bonding is performed. A description on the operation following bonding will be omitted. 
     With the semiconductor device  20  and the method of manufacturing the same according to the embodiments described above, the surface of the second chip  26  is adhered through the adhesive tape  25  such as the LOC tape adhered to the lower surface of the die pad  22 . The first and second chips  24  and  26  can be connected by bonding from the same side (upper side) simultaneously, or without turning the lead frame over as in the conventional case. Thus, the manufacturing cost can be decreased. Since the lead frame need not be turned over during the manufacturing process, wire deformation does not occur during conveyance of the lead frame, and the yield as the semiconductor device  20  can be improved. 
     According to the second to fourth embodiments of the present invention, since the upper surface of the second chip  26  has an LOC structure, the interconnection in the package that connects the first and second chips  24  and  26  to each other can be formed without using TAB tapes, and the cost of the lead frame can accordingly be decreased. 
     According to the semiconductor device lead frame described in the third embodiment, the adhesive tapes  31  are adhered to the lower surfaces of the tab portions  35   a  extending from the die pad  22 , and thereafter the peripheral portion of the die pad  22  is cut, so that the tab portions  35   a  and die pad  22  are separated from each other, thereby forming the inner leads  35  for interconnection in the package that are independent of each other. Therefore, no TAB tapes need be used, and the manufacturing cost of the lead frame can be decreased. 
     According to the semiconductor device lead frame described above in the third embodiment, since the hanging pins  30  and the adhesive tapes  31  that adhere the inner leads  35  for interconnection in the package are adhered to each other, a lead frame in which the inner leads  35  for interconnection in the package are completely independent of each other can be formed. 
     The adhesive tapes  31  such as the LOC tapes are adhered also to the lower surfaces of portions, of the hanging pins  30  supporting the die pad  22 , that overlap the second chip  26 . As a result, the second chip  26  and the hanging pins  30  are prevented from coming into contact with each other. 
     The present invention is not limited to the structures described in the above embodiments, and the shapes, structures, and the like of the respective portions can be modified and changed as required, as a matter of course. 
     For example, the embodiments described above show examples of the structure of the semiconductor device  20  to which the present invention is applied, and the shapes and structures of the respective portions of the semiconductor device  20  can be changed as required. In fine, when the plurality of semiconductor chips  24  and  26  having different chip sizes are to be incorporated in the resin-encapsulated package  20   a  by fixing them on the two surfaces of the die pad  22 , it suffices if the semiconductor chips  24  and  26  can be fixed from one side of the die pad  22 , and if the chips  24  and  26  can be connected to the inner leads  27  by bonding through the bonding wires  28  and  29  from one side of the die pad  22 . 
     As has been described above, with the semiconductor device and the method of manufacturing the same according to the present invention, the surface of the second chip is adhered through the adhesive tape such as the LOC tape adhered to the lower surface of the die pad. The first and second chips can be connected by bonding from the same side simultaneously, or without turning the lead frame over as in the conventional case. Thus, the manufacturing process can be simplified, and the manufacturing cost can be decreased. According to the present invention, since the lead frame need not be turned over during the manufacturing process, inconveniences such as wire deformation do not occur during operation. As a result, the product quality of the semiconductor device is improved, and the yield is improved. 
     According to the present invention, since the upper surface of the second chip has an LOC structure, the interconnection in the package that connects the first and second chips to each other can be formed without using TAB tapes unlike in the conventional case, and the manufacturing cost of the lead frame can accordingly be decreased. 
     According to the present invention, the semiconductor device can be manufactured by using the conventionally used manufacturing apparatus unchanged. 
     According to the semiconductor device lead frame of the present invention, the adhesive tapes are adhered to the lower surfaces of the tab portions extending from the die pad, and thereafter the peripheral portion of the die pad is cut, so that the tab portions and die pad are separated from each other, thereby forming the inner leads for interconnection in the package that are independent of each other. Therefore, no TAB tapes need be used, and the cost of the lead frame can be decreased. 
     According to the present invention, since the adhesive tapes that adhere the hanging pins and the inner leads for interconnection in the package are used, a lead frame in which the inner leads for interconnection in the package are completely independent of each other can be formed. 
     The adhesive tapes such as the LOC tapes are adhered also to the lower surfaces of portions, of the hanging pins supporting the die pad, that overlap the second chip. As a result, the second chip and the hanging pins are prevented from coming into contact with each other.