Patent Publication Number: US-2003230796-A1

Title: Stacked die semiconductor device

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to interconnects of integrated circuits and a method of connecting stacked integrated circuits.  
       [0002] An integrated circuit (IC) die is a small device formed on a semiconductor wafer, such as a silicon wafer. Such a die is typically cut from the wafer and attached to a substrate or base carrier for interconnect redistribution. Bond pads on the die are then electrically connected to the leads on the carrier via wire bonding. The die and wire bonds are then encapsulated with a protective material such that a package is formed. The leads encapsulated in the package are redistributed in a network of conductors within the carrier and end in an array of terminal points outside the package. The terminal points allow the die to be electrically connected with other circuits, such as on a printed circuit board.  
       [0003] With the goal of increasing the amount of circuitry in a package, but without increasing the area of the package so that the package does not take up any more space on the circuit board, manufacturers have been stacking two or more die within a single package. Such devices are sometimes referred to as stacked multichip packages.  
       [0004] Referring to FIG. 1, an enlarged partial side view of a conventional stacked multichip package  10  is shown. The package  10  includes a top die  12 , a bottom die  14  and a substrate  16 . The top and bottom dice  12 ,  14  are electrically connected to the substrate  16  with wires via a wirebonding process. It is common to connect the top die  12  directly to the substrate  16  with long wires (not shown) and the bottom die  14  to the substrate  16  with shorter wires. However, some pads need to be bonded down from the top die  12  to the bottom die  14  then to the substrate  16 .  
       [0005]FIG. 2 is an enlarged top view of a bond pad  22  that can accept two wires and FIG. 3 is an enlarged side view of the bond pad  22  to which wires  18 ,  20  have been wirebonded with ball bonds  24 . In order to accept two wires, the bond pad  22  is elongated. The pad  22  must be elongated not only to accept the two wires, but also so that the capillary (used to perform wirebonding) doesn&#39;t hit the first wirebond when the second wirebond is being performed. Unfortunately, having to provide an elongated bond pad can increase the size of the die.  
       [0006] It would be advantageous to be able to connect two or more wires to a bond pad of a die without having to increase the size of the bond bad. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0007] The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments that are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:  
     [0008]FIG. 1 is an enlarged side view of a conventional stacked multichip package;  
     [0009]FIG. 2 is an enlarged top view of a conventional bonding pad that accepts two wires;  
     [0010]FIG. 3 is an enlarged side view of the bonding pad of FIG. 2 with two wires connected thereto via ball bonding;  
     [0011]FIG. 4 is an enlarged partial side view of an embodiment of a stacked multichip package in accordance with the present invention;  
     [0012]FIG. 5 is an enlarged side view of a bond pad having two wires connected thereto in accordance with the present invention; and  
     [0013]FIG. 6 is an enlarged side view of a bond pad having multiple wires connected thereto in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0014] The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be practiced. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the invention. For simplicity, examples used to illustrate the invention refer only to a stacked die package having two stacked dice. However, the same invention in fact can be applied to other types of packages and to stacked die packages having more than two stacked dice.  
     [0015] Certain features in the drawings have been enlarged for ease of illustration and the drawings and the elements thereof are not necessarily in proper proportion. However, those of ordinary skill in the art will readily understand such details. In the drawings, like numerals are used to indicate like elements throughout.  
     [0016] The present invention is an electrical connection for connecting a plurality of bonding pads. The connection includes a first bonding pad and a bump disposed on the first bonding pad. A first wire is stitch bonded to the bump and a second wire ball is bonded to the stitch bond of the first wire.  
     [0017] The present invention also provides a stacked multichip package having a base carrier, a bottom integrated circuit die and a top integrated circuit die. The base carrier has a top side including a plurality of first bonding pads. The bottom integrated circuit die has a bottom surface attached to the base carrier top side, and an opposing, top surface having a plurality of second bonding pads. The top integrated circuit die has a bottom surface attached to the top surface of the bottom die, and a top surface having a plurality of third bonding pads. A first one of the third bonding pads is electrically connected to a first one of the second bonding pads with a first wire by way of a bump on the first one of the second bonding pads, and a first one of the first bonding pads is electrically connected to the first one of the second bonding pads with a second wire. The first wire is stitch bonded to the bump and the second wire is ball bonded to the stitch bond.  
     [0018] The present invention also provides a stacked multichip package including a base carrier having a top side with a plurality of first bonding pads, a bottom integrated circuit die having a bottom surface attached to the base carrier top side, and an opposing, top surface having a plurality of second bonding pads, and a top integrated circuit die having a bottom surface attached to the top surface of the bottom die, and a top surface having a plurality of third bonding pads. A first electrically conductive bump is disposed on a first one of the second bonding pads. A first wire electrically connects a first one of the third bonding pads to the first one of the second bonding pads by way of the first bump. The first wire is stitch bonded to the first bump with a first stitch bond. A second wire electrically connects a first one of the first bonding pads to the first one of the second bonding pads. The second wire is ball bonded to the first stitch bond.  
     [0019] The present invention also provides a method of electrically connecting a plurality of devices, where each device has a plurality of bonding pads. The method includes the steps of:  
     [0020] disposing a first electrically conductive bump on a first bonding pad of a first device;  
     [0021] stitch bonding a first wire to the first bump with a first stitch bond; and  
     [0022] ball bonding a second wire to the first stitch bond.  
     [0023] Referring now to FIG. 4, a partial side view of an embodiment of a stacked multichip package  40  in accordance with the present invention is shown. The stacked multichip package  40  includes a base carrier or substrate  42 , a bottom integrated circuit die  44  and a top integrated circuit die  46 . The substrate  42  provides an interconnect network for electrically connecting the bottom and top dice  44  and  46  to each other and to other components or devices.  
     [0024] The base carrier  42  has a top side  48  including a plurality of first bonding pads  50 . The bottom integrated circuit die  44  has a bottom surface attached to the base carrier top side  48 , and an opposing, top surface  52 . The top surface  52  includes a plurality of second bonding pads  54 . The top integrated circuit die  46  has a bottom surface attached to the top surface  52  of the bottom die  44 , and a top surface  56  having a plurality of third bonding pads  58 . The base carrier  42 , bottom die  44 , and top die  46  are of a type known to those of ordinary skill in the art and detailed descriptions thereof are not necessary for a full understanding of the invention.  
     [0025] As is known to those of ordinary skill in the art, the base carrier  42 , bottom die  44  and top die  46  are electrically connected with wires wirebonded to the various bonding pads  50 ,  54  and  58 . According to the present invention, multiple wires may be connected to a single bonding pad by stacking the wirebonds in a bump-stitch-ball type sandwich as described below.  
     [0026] As shown in FIG. 4, a first one of the third bonding pads  58  is electrically connected to a first one of the second bonding pads  54  with a first wire  60  by way of a bump  62  on the first one of the second bonding pads  54 , and a first one of the first bonding pads  50  is electrically connected to the first one of the second bonding pads  54  with a second wire  64 . The first wire  60  is stitch bonded to the bump  62  and the second wire  64  is ball bonded to the stitch bond. Of course, it will be understood that the second wire  64  from the first one of the first bonding pads  50  could be stitch bonded to the bump  62  on the first one of the second bonding pads  54  and the first wire  60  from the first one of the third bonding pads  58  could be ball bonded to the stitch bond.  
     [0027] The term ‘wirebonding’ is generally accepted to mean the interconnection, via wire, of chips and substrates. The most frequently used methods of joining the wires to the pads is via either thermosonic or ultrasonic bonding. Ultrasonic wirebonding uses a combination of vibration and force to rub the interface between the wire and the bond pad, causing a localized temperature rise that promotes the diffusion of molecules across the boundary. Thermosonic bonding, in addition to vibration, uses heat, which further encourages the migration of materials. In ball bonding, a capillary holds the wire. A ball formed on one end of the wire is pressed against the face of the capillary. The ball may be formed with a hydrogen flame or a spark. The capillary pushes the ball against the bond pad, and then, while holding the ball against the first pad, ultrasonic vibration is applied, which bonds the ball to the die. Once the ball is bonded to the die, the capillary, which is still holding the wire, is moved over a second bonding pad to which the first pad is to be electrically connected. To form a stitch bond, the wire is pressed against the second pad, forming a wedge-shaped bond. Once again, ultrasonic energy is applied until the wire is bonded to the second pad. The capillary is then lifted off the bond, breaking the wire. Both stitch bonding and ball bonding are well known by those of skill in the art.  
     [0028] Referring now to FIG. 5, an enlarged side view of the second bond pad  54  having the first and second wires  60 ,  64  connected thereto in accordance with the present invention is shown. FIG. 5 highlights the electrical connection for connecting ones of the first, second and third bonding pads. The bump  62  may be larger, smaller, or the same size as the ball  66  and in most cases, will probably be the same size as the ball  66  as they are formed in a similar manner. However, in one embodiment of the invention, the bump  62  is bigger than the ball  66  of the ball bond to ensure that the ball  66  and the stitch bond sit completely on the bump  62 . The diameter of the bump  62  and the ball  66  depend in large part on the diameter of the wire from which they are formed. For example, for 25 um (1 mil) wire, the bump  62  can be controlled to be between about 48-55 um and the ball  66  will be between about 45-55 um. In one embodiment of the invention, a 20 um gold wire was wirebonded to a 76 um×76 um pad. The bump formed on the pad was about 42 um and the ball had a diameter of about 35 um.  
     [0029] The wires  62 ,  64  may be formed of any electrically conductive metal or combination of metals, such as are known by those of skill in the art. Suitable bond wires typically comprise a conductive metal such as copper or gold and may be either fine wires (&lt;50 um in diameter) or heavy wires (&gt;50 um in diameter). The bump  62  preferably comprises the same materials as the wires  62 ,  64  and is formed or disposed on the bonding pad in the same manner that the ball bond is formed, such as with a hydrogen flame or a high voltage electrical spark. More particularly, a ball is formed on one end of the wire in the wirebonder. The formed ball is pressed against the face of the wirebonder capillary. The capillary pushes the ball against the die bond pad, and then, while holding the ball against the pad, ultrasonic vibration is applied, which bonds the ball to the die pad. Once the ball is bonded to the die pad, the wire above the bonded ball  62  is cut-off by clamping the wire above the capillary while the capillary is being lifted up. The wire area above the ball, which is the weakest spot, will give way, leaving only the bump  62 . In short the bump  62  is formed as if a full bond cycle for first bond (the ball) and the stitch was completed but no wire looping was performed in between the ball and stitch bond.  
     [0030]FIG. 6 is an enlarged side view of a bond pad  70  having multiple wires connected thereto in accordance with the present invention. More particularly, the bond pad  70  has a first bump  72  disposed on its surface. A first wire  74  is stitch bonded to the first bump  72  with a first stitch bond. A second bump  76  is formed on the first stitch bond. A second wire  78  is stitch bonded to the second bump  76  with a second stitch bond. A third bump  80  is formed on the second stitch bond and a third wire  82  is stitch bonded on the third bump  80  with a third stitch bond. A fourth bump  84  is then formed on the third stitch bond and a fourth wire  86  is stitch bonded to the fourth bump  84  with a fourth stitch bond. A fifth wire  88  is then ball bonded on the fourth stitch bond with a ball bond  90 . All of the bumps  72 ,  76 ,  82  and  84  are generally of the same size, as wirebonding machines are capable of high accuracy. Current wirebonders are also capable of keeping the bumps on center. The bumps  72 ,  76 ,  80  and  84  are formed of the same material as the wires  74 ,  78 ,  82 ,  86  and  88  and disposed on the pad  70  and stitch bonds, respectively, with the wirebonder as described above, using a hydrogen flame or a spark. No modifications to the capillary are required.  
     [0031] The present invention further provides a method of electrically connecting a plurality of devices, where each of the devices has a plurality of bonding pads. The method includes the steps of disposing a first electrically conductive bump on a first bonding pad of a first device, stitch bonding a first wire to the first bump with a first stitch bond, and then ball bonding a second wire to the first stitch bond. Further wires may also be stitch bonded between the first stitch bond and the ball bond. For example, a third wire may be electrically connected to the bonding pad by disposing a second electrically conductive bump on the first stitch bond and stitch bonding the third wire to the second bump with a second stitch bond. The second wire is then ball bonded to the second stitch bond. The sandwich (bump-stitch-ball) type interconnects formed with the foregoing method have been found to provide stronger bonds than a single stitch bond. Wire pull and wire peel have been within desired tolerances.  
     [0032] The description of the preferred embodiments of the present invention have been presented for purposes of illustration and description, but are not intended to be exhaustive or to limit the invention to the forms disclosed. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. For example, the present invention is not limited to a package with two stacked dice, but can be applied to a package with multiple stacked dice. Further, the present invention is not limited to stacked devices, but is applicable to all wire bonded package types, including but not limited to BGA, QFN, QFP, PLCC, CUEBGA, TBGA, and TSOP. The present invention can be applied in other applications, such as to replace a very long wire used to connect two distant bond pads. That is, to replace a very long wire where two bonds to one pad is required. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but covers modifications within the spirit and scope of the present invention as defined by the appended claims.