Patent Publication Number: US-6657870-B1

Title: Die power distribution system

Description:
FIELD OF THE INVENTION 
     The present invention relates to power mesh designs for semiconductor devices, and more particularly to a power distribution system for high power consumption, high pin-count chips designed for use in wire-bond and flip-chip packages. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits are typically packaged before they are used as other components as part of a larger electronic system. A ball grid array (BGA) is a popular surface mount chip package that uses a grid of solder balls to connect the package to a printed circuit board. The chips within the package may be wire bond or flip-chips. Wire-bond BGA packages are constructed with die mounted on a substrate with bond pads on the die connected to conductive lines or traces on the surface of the substrate. Flip-chips have solder balls placed on the surface of the chip, and the chip is “flipped” over onto the substrate and connected via the solder balls. One area of concern for BGA packages is power distribution to the die. 
     FIG. 1 is a top view of a conventional power mesh for redistributing external power across a die. A conventional power distribution system typically uses two metal layers to create a power mesh  10  across a die  12  to supply external power and ground (referred to as VDDCORE and VSSCORE, respectively) to rows of cells  16  within the die  12 . Power and ground trunks  12  are placed within the top-metal layer of the die  12  and are usually patterned perpendicular to the rows of cells  16  to permit vias  20  to be placed along the length of the cell rows  16  at regular intervals. In order to provide uniform distribution across the die  12 , the metal layer below the top layer also includes power and ground trunks  18 , which are patterned parallel to the cell rows  16 . This second layer of trunks  18  is connected to the top metal layer trunks  14  using vias  22 . 
     Referring now to FIG. 2, the traditional power mesh system of FIG. 1 is shown in a wire bond ASIC implementation. In the wire bond implementation, an even distribution of VDDCORE and VSSCORE bond pads  30  and  32  is required around the periphery of the die  12  for receiving external power and ground, respectively. I/O signal bond pads  34  are also placed along the periphery of the die  12  for connection with I/O signal lines  36 . 
     Although the traditional power mesh system is well automated within design tools and also provides uniform power distribution across die, the conventional power mesh system includes several drawbacks. First, the power mesh  10  requires at least two metal layers to pattern the perpendicular VDDCORE and VSSCORE trunks  14  and  18 . Unfortunately, the layer below the top metal layer is a routing resource that could be used for signal routing rather than for power routing, which could result in smaller die  12  sizes. 
     Second, the interior of the die  12  may experience a voltage drop due to the length of the VDDCORE and VSSCORE trunks  14  and  18 . For example, assuming that the external power source is 5 V, then the die  12  may experience a 5V−10% drop at the center. 
     Third, requiring uniform placement of VDDCORE and VSSCORE bond pads  30  and  34  is not ideal from an I/O placement perspective because the uniform placement of the power bond pads require that more I/O signal bond pads  34  be placed towards the corners of the die  12 . When I/O signals are forced to the corners of the die  12  in order to connect to the signal I/O bond pads  34 , a mismatch between bond wire length and package trace lengths is created, which may cause skew on wide I/O signal lines  36 . 
     The traditional power mesh system  10  also has disadvantages when used in flip-chip implementations, as shown in FIG.  3 . FIG. 3 is a top view of a power mesh  10 ′ used in a conventional flip-chip ASIC implementation. FIG. 3 is a more detailed view showing that each trunk on the top metal layer actually includes a separate VDDCORE trunk  14   a  and VSSCORE trunk  14   b , and each trunk on the layer beneath the top layer also includes a VDDCORE trunk  18   a  and VSSCORE trunk  18   b.    
     As stated above, the top metal layer in flip-chips is reserved for I/O to flip-chip solder bump connections, which include VDDCORE bumps  40  and VSSCORE bumps  42 . However, the traditional power mesh  10 ′ also uses the top metal layer. Therefore, when the traditional power mesh  10 ′ is used with a flip-chip, routing on the top metal layer becomes very congested. For core limited designs, use of a two metal layer power mesh  10 ′ constrains routing. 
     In addition, the VDDCORE bumps  40  and VSSCORE bumps  42  are not necessarily evenly distributed across the die  12 ′; they are usually located on the center of the die  12 ′ and the power mesh  10  must distribute current from the bumps  40  and  42  to the corners of the die  12 ′. Because via connections  22 ′ are used to carry current from the center of the die  12 ′ towards the corners of the die  12 ′ in a staircase fashion across the orthogonal mesh power mesh  10 ′, additional resistance and routing blockages may be introduced. Furthermore, potential IR drops may also occur if there are large current sinks  44  at the die corners. 
     Accordingly, what is needed is an approved single-layer power mesh that achieves symmetry in power distribution both within the die and through the power pads. The present invention addresses such a need. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system and method for distributing external power across a die, which has horizontal and vertical centerlines. The system and method include providing a power mesh that includes a plurality of V-shaped trunks patterned as concentric diagonal trunks extending from the horizontal and vertical centerlines of the die towards the periphery of the die. 
     According to the method system disclosed herein, because the trunks are routed diagonally across the die, all the power bond pads can be connected without the need for a second layer, thereby providing a single-layer power mesh. The single-layer power mesh of the present invention achieves symmetry in power distribution both within the die and an even distribution of current flow. In addition, the single-layer power mesh frees a routing resource for signal routing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a conventional power mesh on a die for redistributing external power across the die. 
     FIG. 2 is a top view of a traditional power mesh system with wirebond I/O placement limitations. 
     FIG. 3 is a top view of a power mesh in a conventional flip-chip ASIC implementation. 
     FIG. 4 is a top view of a V-shaped power mesh for redistributing external power across a die in accordance with a preferred embodiment of the present invention. 
     FIG. 5 is a block diagram illustrating the V-shaped power mesh system in a wire bond implementation. 
     FIG. 6 is a top view of a single-layer V-shaped power mesh that routes V-shaped trunks in a stair-step arrangement across the die. 
     FIG. 7 is a diagram illustrating the V-shaped power mesh in a flip-chip implementation. 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to providing a power mesh for redistributing external power across a die. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     The present invention comprises using only one layer, preferably the top metal layer, for a power mesh, while achieving an even distribution of the current flow and also freeing layers beneath the top metal layer as a routing resource. Rather than providing the power mesh as two perpendicular layers of trunks connected between bond pads on opposite sides of the die, the present invention provides a single-layer power mesh built from concentric diagonal trunks extending from the die&#39;s centerlines toward the sides of the die. 
     Referring now to FIG. 4, a top view of a V-shaped power mesh  50  for redistributing external power across a die is shown in accordance with the first preferred embodiment of the present invention. The die  51  is shown having four-sides and a horizontal centerline  52  and a vertical centerline  54 . According to the present invention, the V-shaped VDDCORE trunks  56  and VSSCORE trunks  58  extend from the centerlines  52  and  54  to the four sides of the die  51  in a rotationally symmetric pattern. The V-shaped VDDCORE trunks  56  and VSSCORE trunks  58  regularly intersect cell rows  60  where vias  62  may be used for connection. The V-shaped trunks  56  and  58 , which extend to all four sides of the die  51 , provide distributed coverage across the die, including the die corners. In addition, the trunks  56  and  58  are routed diagonally across the die  51 , all the power bond pads can be connected without the need for a second layer. 
     FIG. 5 is a block diagram illustrating the V-shaped power mesh system in a wire bond implementation. In a preferred embodiment, the V-shaped VDDCORE and VSSCORE trunks  56  and  58  connect to VDDCORE and VSSCORE power pads  64  and  66 , respectively, that are located on substantially the same side of the die  51  to provide robust power connections to the corners of the die  51  where IR drop has traditionally been a problem. Accordingly, the present invention requires only a few VDDCORE and VSSCORE bond pads  64  and  66 . Moving toward each corner of the die  51 , the number of VDDCORE and VSSCORE bond pads  64  and  66  must be progressively increased to handle the increasingly longer trunk lengths. In this manner, the VDDCORE and the VSSCORE bond pads  64  and  66  are progressively biased towards each corner of the die  51 , allowing signal I/O bond pads  68  to occupy most of the center of the die&#39;s periphery for connection with signal I/O bond wires  70 . Biasing the I/O bond pads  68  towards the center results in a fewer number of mismatched I/O signals across the die  51  and reduces skew for wide I/O bond wires  70 . 
     Some types of fabrication tools may be incapable of patterning straight-line diagonal metal layers. Therefore, as shown in FIG. 6, the second preferred embodiment of the present invention provides a single-layer V-shaped power mesh  90  that routes the V-shaped trunks  92  in a stair-step arrangement across the die. Alternatively, the stair-step trunks may be patterned using two metal layers; one layer may be used for the vertical segments of the stair-step, and the second layer for the horizontal segments of the stair-step. 
     FIG. 6 also illustrates an IR drop map for a 12.2×12.2 mm wirebond implementation resulting from the use of the single layer V-shaped power mesh  90 . An IR drop analysis was performed based on 1.8V core voltage at 11 watts using 76 VDDCORE and 76 VSSCORE bond pads. The worst-case IR drop occurred at the die center, but was within a 5% margin of the 1.8V core supply and could be further reduced with a much thicker redistribution layer. 
     FIG. 7 is a diagram illustrating a V-shaped power mesh  100  in a flip-chip implementation. An ASIC die  102  is shown, which includes VDDCORE flip-chip bumps  106  and VSSCORE flip-chip bumps  108 . The flip-chip bumps  106  and  108  are located primarily in the center of the die  102  as normal, but the VDDCORE flip-chip bumps  106  are placed along the paths of the VDDCORE trunks  110 , and the VSSCORE flip-chip bumps  108  are placed along the paths of VSSCORE trunks  112 . 
     For flip-chip ASICs, the V-shaped VDDCORE and VSSCORE trunks  110  and  112  provide a more direct connection to each corner of the die  100 , where VDDCORE and VSSCORE bumps  106  and  108  are not typically present. The shorter path to the die corner reduces the path resistance and therefore the voltage drop. 
     The V-shaped power mesh of the present invention will achieve an even distribution of the current flow over power contact points of the die, such as power pads and flip-chip bumps, and avoids potential electromigration issues. Also, because the trunks are routed diagonally, the V-shaped power mesh requires only one layer of metal to connect all the power bond pads. Since the layer underneath the top metal layer is no longer used for the power mesh, an additional routing resource will be freed for chip level routing. In conventional two-layer power mesh designs, up to 20% of the layer beneath the top layer is used to support the power mesh. In the present invention, this same amount is freed for signal routing, resulting in a die size reduction. 
     A power distribution method and system has been disclosed. The present invention has been described in accordance with the embodiments shown, and one of ordinary skill in the art will readily recognize that there could be variations to the embodiments, and any variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.