Abstract:
The present invention provides a structure, apparatus, and method for wire bonding in which a first wire bond is formed between first and second components, a second wire bond is formed between the second component and a third component such that the second wire bond is in electrical communication with the first wire bond, wherein the first and second wire bonds are connected to said first and second components, respectively, using ball bonding.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to for wire bonding, and more particularly, to a method and apparatus for wire bonding multiple semiconductor integrated circuit chips.  
         BACKGROUND OF THE INVENTION  
         [0002]    The continuing trend in the semiconductor and integrated circuit industries is to develop and manufacture smaller components. This trend has resulted in integrated circuits and semiconductor devices having higher density due to an increased number of components coexisting in smaller physical areas. This downsizing has directly affected the location, number, and size of bond pads for electrical connections for these devices.  
           [0003]    Wire bonding techniques have been developed to accommodate smaller bond pad sizes as well as the stacking of multiple chips in an integrated circuit package. However, decreased size and fewer locations of bond pads on various layers of multiple chips presents a different bonding problem. There exists a need, therefore, for a reliable wire bonding method which provides wire bonds among chips on different layers of a stacked chip assembly, as well as for the resulting wire-bonded assembly.  
         BRIEF SUMMARY OF THE INVENTION  
         [0004]    The present invention provides, in one embodiment, a wire bonded structure having a substrate, a lower chip and an upper chip, a first wire bond formed from the substrate to the lower chip, a second wire bond formed from the lower chip to the upper chip and electrically connected to the first wire bond, wherein the first and said second wire bonds are configured such that an imaginary line drawn between the endpoints of the first wire bond and an imaginary line drawn between the endpoints of the second wire bond form an angle therebetween.  
           [0005]    In another aspect the present invention provides a wire bonding apparatus for forming a first conductive bump on a first conductive surface, a first ball bond on a second conductive surface, a first wire bond from the first ball bond to the first conductive bump, a second conductive bump on a third surface, a second ball bond on the second conductive surface in electrical communication with the first conductive bump, and a second wire bond from the second ball bond to the second conductive bump.  
           [0006]    The present invention also provides a method for forming such a structure by forming a first wire bond between first and second components, forming a second wire bond between the second component and a third component such that the second wire bond is in electrical communication with the first wire bond, and connecting the first and second wire bonds to the first and second components using ball bonding.  
           [0007]    Additional features and advantages of the present invention will be more clearly apparent from the detailed description which is provided in connection with accompanying drawings which illustrate exemplary embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic view of a typical wire bonding apparatus;  
         [0009]    [0009]FIG. 2A is a side view of a capillary for forming wire bonds which is used in the present invention;  
         [0010]    FIGS.  2 B- 2 D illustrate steps in forming a wire bond using the FIG. 2A capillary;  
         [0011]    FIGS.  3 A- 3 B illustrate steps in forming another type of wire bond using the FIG. 2A capillary;  
         [0012]    FIGS.  4 A- 4 B illustrate a prior art method of forming a wire bond among multiple chips of a semiconductor assembly;  
         [0013]    FIGS.  5 A- 5 B show a wire bonding method of forming a wire bond among multiple chips in accordance with an embodiment of the present invention;  
         [0014]    [0014]FIG. 6 is a top view of the wire bonding structure of FIG. 5B;  
         [0015]    [0015]FIG. 7 is a diagram of the process steps of the present invention; and  
         [0016]    [0016]FIG. 8 is a perspective view of FIG. 6. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    In the following detailed description, reference is made to various specific embodiments in which the invention may be practiced. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be employed, and that structural and procedural changes may be made without departing from the spirit or scope of the present invention.  
         [0018]    Before discussing the invention in detail some conventional wire bonding techniques will be discussed with reference to FIGS.  1 - 4 B. Referring now to the drawings, where like elements are designated by like reference numerals, FIG. 1 depicts a wire bonding apparatus generally designated by numeral  1 . The wire bonding apparatus may have a wire bonding device  2 , such as a capillary, which is controllably positioned relative to workpieces  3  on top of a support surface  4 . The bonding device  2  is capable of forming wire bonds at a plurality of bonding positions on workpieces  3 .  
         [0019]    The wire bonding apparatus  1  further comprises a drive unit  5 , such as a motor, for selectively moving control arm  6  which in turn moves the bonding device  2  in any direction represented by multiple arrows  7 . The wire bonding apparatus may also comprise a measuring device  8  for measuring movements of the wire bonding device  2 , and a controller  9  for controlling the drive unit  5 . The foregoing description of a typical wire bonding apparatus is illustrative only and in no way meant to be restrictive in order to practice the present invention.  
         [0020]    [0020]FIG. 2A depicts a capillary  10  which can be used for wire bonding device  2  of the wire bonding apparatus shown in FIG. 1. The capillary  10  has an internal channel  12  to accommodate flow of material, and an opening  14  to introduce the material to an intended surface.  
         [0021]    The capillary  10  can be used to form a conductive bump on a surface as depicted in FIGS.  2 B- 2 D. In use, a conductive wire  20  is fed through the channel  12  and out of the opening  14 . The wire  20  is preferably a gold wire, however, any suitable conductive material can be substituted. A ball  22  is formed at the tip of the wire  20  by energy generated from an electric discharge of a torch electrode  25 , or by heating the tip of the capillary  10 . Other methods of forming the ball  22  can also be utilized. The size of the ball  22  can be controlled by varying hardware and software of the wire bonding apparatus.  
         [0022]    After the ball  22  is formed, the capillary  10  is positioned above a desired location on a bonding surface  26 . The ball  22  is then forced downward to the surface  26  by downward movement of the capillary  10 , thereby causing ball  22  to deform into a mass  22   a . The downward force of the capillary  10  can be combined with ultrasonic energy to create a bond between the ball  22  and the bonding surface  26 . Thereafter, as shown in FIG. 2D, the capillary  10  can be moved away from the surface  26  to leave behind a conductive bump  22   b  by cutting wire  20 . This technique is generally referred to as ball bonding. Alternatively, with wire  20  still attached to the conductive bump  22   b , the capillary  10  can be moved to a second position on the bonding surface  26  (or another bonding surface) to form a wire bond connection between the second bonding position and bump  22   b.    
         [0023]    Another bonding technique which can be performed by the capillary  10  is called wedge or stitch bonding, and is illustrated in FIGS. 3A and 3B. The capillary  10  is heated and lowered onto the bonding surface  26  to a distance approximately equal to the thickness of the wire  20 . The capillary  10  is then moved relative to the bonding surface  26  as the wire  20  is fed to leave behind a molten linear wire bond section  28  having a bond length  29 . The capillary  10  can be used for both ball bonding and stitch bonding.  
         [0024]    [0024]FIGS. 4A and 4B show an integrated circuit assembly comprising components  30 ,  32 , and  34 . These components can be, for example, a substrate  30 , a bottom chip  32 , and an upper chip  34 . A connection between conductive areas, or bonding pads  38 , of components  30 ,  32 , and  34  can be made utilizing the wedge or stitch bonding technique discussed above. This bonding technique requires formation of linear wire bond sections  28  having lengths  29  (as shown in FIG. 3B) on each bonding pad  38  of the integrated circuit components  30 ,  32 ,  34 . Because each stitch wire bond requires formation of a linear wire bond section  28 , extending the wire  20  from one bonding pad to another bonding pad has to be done through substantially a linear motion of the capillary  10  because any significant angling of the capillary  10  would likely result in breakage of the wire  20 .  
         [0025]    For example, with reference to FIG. 4B, a stitch wire bond formed on bonding pad  38   a  would dictate the next stitch wire bond to be formed on bonding pad  38   b . If, in use, it is desirable to extend the wire  20  from bonding pad  38   a  to form the next stitch wire bond on bonding pad  38   c , the wire  20  (represented by dashed lines  20   c ) is susceptible to breaking. Additionally, having made the stitch wire bond having the linear section  28  on bonding pad  38   a , a typical wire bonding apparatus cannot reliably make the required turn to reach bonding pad  38   c . Consequently, the shape of the entire wire bond is limited to being substantially linear as represented by imaginary line  40  in FIG. 4B.  
         [0026]    Thus, due to the need for linear bond sections  28  of stitch wire bonds, and the miniature, often microscopic nature of semiconductor and integrated circuit components and associated closely spaced bonding pads, it is very difficult to form a wire bond between the components  30 ,  32 ,  34  particularly if target bonding pads are angled away from each other.  
         [0027]    The inventive method described herein allows for greater flexibility in forming wire bonding connections between multiple tiered surfaces and permits greater wire bonding angles between bond pads at different tiers of a multi-tiered assembly.  
         [0028]    The invention will now be described with reference to FIGS. 5A, 5B through FIG. 8. Referring now to FIG. 5A, a wire bonding machine having an apparatus such as capillary  10  is used in an exemplary embodiment of the invention to first deposit a conductive bump  50  onto a bonding pad of the lower chip  32 . The capillary is removed from contact with the bonding pad of the lower chip without drawing a wire  20  from bump  50 . The capillary then bonds a ball  52  to the substrate  30 , and, without cutting the wire  54 , extends the wire  54  from the ball  52  to the bump  50 . The bump  50 , which is in place before wire  54  is bonded to lower chip  32 , acts as a cushion to prevent damage to the chip  32  when wire  54  is bonded thereto. Then, with reference to FIG. 5B, the machine causes the capillary  10  to bond a bump  56  onto the upper chip  34 . Thereafter, the machine causes the capillary  10  to bond a ball  58  on top of the bump  50  of the lower chip  32 , and, without cutting the wire  60 , extend wire  60  from the ball  58  to the bump  56 .  
         [0029]    A side view and top view of one possible resultant structure constructed according to the method just described can be seen in FIG. 5B and 6. FIG. 6 shows stacked integrated circuit components  30 ,  32 ,  34  having several wire bonds installed. The upper portion of the wire bond, wire  60 , connects the lower chip  32  to the upper chip  34 , while the lower potion of the wire bond, wire  54 , connects the substrate  30  to the lower chip  32 .  
         [0030]    As can be appreciated from FIGS. 5B and 6, the lower portion of the bond wire  54  can be easily offset linearly from the upper portion of the bond wire  60  resulting in a stand off bonding connection between stacked chips and/or substrates. In FIG. 5B, imaginary line  55  is drawn along the longitudinal axis of the upper bond wire  60  and imaginary line  57  is drawn along the longitudinal axis of lower bond wire  54 . Alternatively, imaginary lines  55  and  57  can be drawn between endpoints of bond wires  60  and  54 . Arc  59 , drawn between lines  55  and  57  represents an angle by which the upper and lower bond wires  60 ,  54  can be offset in the vertical plane.  
         [0031]    Referring now to FIG. 6, imaginary lines  70  are drawn along the longitudinal axis of the upper bond wire  60 , and imaginary lines  72  are drawn along the longitudinal axis of the lower bond wire  54 . Arcs  74 , drawn between lines  70  and  72 , represent the angles by which the two portions of the wire bond can be offset in the horizontal plane to allow for improved capability in connecting semiconductor and integrated circuit components. A relatively linear wire bond  64 , having an imaginary line  66  drawn along its longitudinal axis, can also be produced by the herein described method, and is shown for the purpose of comparison.  
         [0032]    The foregoing wire bonding procedure can be performed by incorporating into a high-level software program the sequence of process steps of the present invention. Such a sequence of steps is illustrated in FIG. 7, and a software program containing the steps of FIG. 7 can be loaded into and executed by the controller  9  of the wire bonding apparatus of FIG. 1.  
         [0033]    [0033]FIG. 8 shows a perspective view of the substrate  30 , lower chip  32 , and upper chip  34  having multiple wire bonds installed using the apparatus and method of the present invention. It can appreciated from FIG. 8 that the technique of the present invention allows for wire bonds between stacked layers of a device to be formed in various planes and at various angles to make possible connections between bond pads in various areas of the completed assembly.  
         [0034]    While exemplary embodiments of the invention have been described and illustrated, it should be apparent that many modifications can be made to the present invention without departing from its spirit and scope. For example, while connections between stacked components have been described, the present invention is equally applicable for interconnecting components arranged side-by-side, in the same plane, or in other configurations. Accordingly the invention is not limited by the foregoing description or drawings, but is only limited by the scope of the appended claims.  
         [0035]    What is claimed as new and desired to be protected by Letters Patent of the United States is: