Abstract:
A semiconductor package structure includes: a substrate comprising a plurality of power supply balls on a first surface of the substrate, a first metal conductor on a second surface of the substrate and at least one via coupling a power supply ball to the first metal conductor of the substrate; a die, comprising a plurality of bond pads on a first surface of the die, a first metal conductor on a second surface of the die and at least one via coupling a bond pad to the first metal conductor of the die; and a plurality of first wire bonds for coupling the first metal conductor of the substrate to the first metal conductor of the die.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor package, and more particularly, to a semiconductor package structure that has low power supply inductance. 
     2. Description of the Prior Art 
     Memory, such as DRAM and logic devices are commonly used in modern computers and electronic products. These memory devices are usually packaged as a semiconductor chip. An exemplary DRAM package consists of a semiconductor DRAM die mounted under a single layer substrate. This is commonly known as a Board-on-Chip (BOC) design. One side of the die comprises a plurality of bond pads and can be referred to as a circuit side. The substrate also has a circuit side comprising a plurality of internal conductive traces and a plurality of external contacts in the form of power supply soldering balls, which communicate electrically with the traces on the substrate. The DRAM package is formed by bonding the circuit side of the die to the backside (non-circuit side) of the substrate. Traces are then coupled to the bond pads using wire bonds. Other package configurations are Chip-on-Board (COB) designs and Fine Ball Grid Array (FBGA) designs, both also have the same concept of power supply balls coupling to bond pads. In the following description, reference will be made to the BOC design for brevity. 
     Please refer to  FIG. 1A ,  FIG. 1B  and  FIG. 1C , which represent, respectively, a top view, a first side view and a second side view of a conventional BOC DRAM package  10 .  FIG. 1A  shows a package substrate  15  comprising a number of power supply balls  22 ,  24 ,  26  and a DRAM die  30  (represented by the dashed outline). The substrate  15  is rectangular in shape with a hollow middle section that exposes the die  30  underneath. The die  30  has a number of bond pads  32 ,  34 ,  36  located on the centre section of the die  30  which are exposed through the hollow middle section of the package substrate  15  as shown in  FIGS. 1A and 1C . The first side view, shown in  FIG. 1B , shows the package substrate  15  bonded to the die  30  from an end perspective and also shows the power supply balls including the power supply ball  26  on the circuit side of the package substrate  15 . In  FIG. 1C , the wire bonding between the package substrate  15  and the bond pad  32  on the die  30  is shown. Wire bond  42  couples the bond pad  32  to conductive traces (not shown) in the package substrate  15 , which in turn are coupled to the power supply ball  22 . The wire bonding is only from the circuit side of the substrate  15  to the circuit side of the die  30 . 
     Please refer to  FIG. 2  for a circuit-level illustration of the connections between the bond pads and the power supply balls. The diagram shows a single output circuit for simplicity, comprising an output driver coupled between two power supply balls  24 ,  26 , respectively coupled to VSSQ and VDDQ with the output voltage DQ_Out being output through a third power supply ball  22  which is coupled between VDDQ_EXT and VSSQ_EXT (ground). Power supply balls  24 ,  26  are respectively coupled to bond pads  34 ,  36 , and power supply ball  22  is coupled to bond pad  32 . The current is pulled and pushed between VSSQ and VDDQ, and the switching results in a considerable amount of inductive Simultaneous Switching Output (SSO) Noise on the power supply rails. 
     One method to reduce this inductance is to increase the thickness of the bonding wires. This is not very effective, however, as the number of bonding wires that can be used is limited by the space available in the package. Modern designs tend to have a congested routing environment; using thicker wires would therefore involve increasing the size of the substrate, which is unlikely to provide an effective solution to high speed SSO noise reduction. Modern high speed I/O package designs require a minimized power supply inductance in conjunction with a small substrate size. 
     SUMMARY OF THE INVENTION 
     With this in mind, the present invention aims to provide a DRAM package that can reduce the amount of inductance as well as providing a greater surface area available for bonding. 
     A semiconductor package structure according to an exemplary embodiment of the present invention comprises: a substrate comprising a plurality of power supply balls on a first surface of the substrate, a first metal conductor on a second surface of the substrate and at least one via coupling a power supply ball to the first metal conductor of the substrate; a die, comprising a plurality of bond pads on a first surface of the die, a first metal conductor on a second surface of the die and at least one via coupling a bond pad to the first metal conductor of the die; and a plurality of first wire bonds for coupling the first metal conductor of the substrate to the first metal conductor of the die. 
     A method of forming a semiconductor package according to an exemplary embodiment of the present invention comprises: providing a substrate comprising a plurality of power supply balls on a first surface of the substrate; plating a first metal conductor on a second surface of the substrate; forming at least one via that couples a power supply ball to the first metal conductor of the substrate; providing a die comprising a plurality of bond pads on a first surface of the die; plating a first metal conductor on a second surface of the die; forming at least one via that couples a bond pad to the first metal conductor of the die; and coupling the first metal conductor of the substrate to the first metal conductor of the die. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a top view diagram of a prior art die and package. 
         FIG. 1B  is a first side view diagram of a prior art die and package. 
         FIG. 1C  is a second side view diagram of a prior art die and package. 
         FIG. 2  is a circuit level diagram of the prior art die and package shown in  FIG. 1 . 
         FIG. 3  is a bottom view diagram of a DRAM package according to an exemplary embodiment of the present invention. 
         FIG. 4  is a diagram showing three views of the die in  FIG. 3 . 
         FIG. 5  is a side view diagram of the DRAM package shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention therefore aims to provide a semiconductor package structure that has reduced inductance while still maintaining the small size of the substrate. Various embodiments for achieving this aim will be detailed in the following. 
     In this disclosure, the invention will be discussed with reference to reducing the inductance on the ground power supply rail (the VSSQ). The method for reducing the inductance on the VSSQ power supply rail involves establishing a common ground reference point, which eliminates the need for an isolated ground point or multiple ground points. As ground will always remain the same, it is more practical to have one common ground reference while still allowing different values for VCC/VDDQ; i.e. if there is a change in current demand, a different power supply ball can be used as VDDQ while VSSQ remains at the same potential. It should be obvious to one skilled in the art, however, that the following methods and principles can equally be applied to VDDQ without departing from the spirit of the present invention. Reducing inductance at VSSQ is a preferred embodiment and not a limitation. 
     In the following, the spirit of the invention will be described with reference to a DRAM package structure, but one skilled in the art will easily be able to apply the concepts set out in this disclosure to other memory devices such as logic devices. Additionally, the accompanying drawings illustrate a BOC DRAM package design, but one skilled in the art will understand that the following concepts can also be applied to other package designs such as COB and FBGA designs. 
     As set out in the above, the present invention introduces a common ground reference to a semiconductor package such as a DRAM. This is achieved by plating the back side of the die and the back side of the package substrate with metal or any types of conductive plating. These back side metal plates/back side conductors are then coupled together such that the entire package has one common power supply reference plate. This strong common ground reference means that, during SSO switching between VSSQ and VDDQ, the voltage drop will be minimized and therefore the inductance at the VSSQ power supply rail is reduced. The packaging means of providing the common ground reference has the added advantage of increasing the amount of space available for wire bonding between the die and the package substrate. 
     Please refer to  FIG. 3 , which is a bottom view of a semiconductor package  100  consisting of a package substrate and die, wherein both the back side of the package substrate and the back side of the die have been plated with a conductor (gold, for example). The package substrate back side conductor  119  and the die back side conductor  132  are coupled by means of VSS wire bonds  145 . Therefore, a common ground reference plate is formed for both the package substrate and the die. These VSS wire bonds  145  are not coupled to the circuit side of the die or the circuit side of the package substrate. 
     In the conventional art shown in  FIGS. 1A ,  1 B and  1 C, wire bond  42  coupled the bond pad  22  on the die  30  to substrate traces on the package substrate  15 , which were in electrical communication with the power supply ball  22 . Coupling occurred from the circuit side of the die  30  to the circuit side of the package substrate  15  only. The DRAM package  100  of the present invention uses a back-to-front (i.e. back side to circuit side) coupling scheme to both enable the common ground reference for the DRAM package  100  and increase the amount of area available for wire bonding. In order to allow this back-to-front coupling, vias are introduced in both the package substrate and the die. 
     Please refer to  FIG. 4 , which are diagrams illustrating top, side and bottom views of the die  130  of the DRAM package  100 . As is shown, the circuit side of the die  130  has a number of bond pads including bond pad  44  which are located on the centre section of the die  130 . These are illustrated in the top view diagram. The present invention introduces through silicon vias (TSV) which connect the circuit side of the die  130  to the backside of the die  130 , including TSV  150  which couples to bond pad  44  for coupling bond pad  44  to ground. When these vias are formed directly below the bond pads, as shown in  FIG. 4 , they therefore couple the bond pads to the back side conductor  132  (VSSQ in this embodiment). Please note that through silicon vias are a technology commonly used in semiconductor packages and there are many methods that can be used to form them: for example, plasma etch technology which is often used for deep silicon etching in memory devices. The formation of the TSVs is not limited to a particular process. Furthermore, the TSVs are not limited to be created at a certain stage in the manufacturing process; both via-last and via-first processes are well-known to those skilled in the field of semiconductor technology. 
     The bottom view in  FIG. 4  shows the back side of the die  130  that has been plated to form the back side conductor  132 , as well as showing the location of the through-silicon-vias (which correspond to the location of the bond pads in this embodiment). The through-silicon-vias couple the bond pads directly to ground, whereas in the prior art the bond pads are coupled to ground via wire bonds, the substrate traces and power supply balls. Utilizing the back side of the die as the common ground reference for the package  100  and coupling the bond pads internally to the back side conductor  132  therefore reduces the number of wire bonds that are required on the circuit side of the die  130 , resulting in a greater area of the die being available for coupling bond pads to VDDQ. Further, the use of the common ground reference provides better current return paths and loop inductances to handle much higher speed I/O designs. 
     As shown in the circuit diagram of  FIG. 2 , power supply ball  24  is coupled to ground (VSSQ). In the package  100  of the present invention, the back side conductor  119  (shown in  FIG. 3 ) of the package substrate functions as the common ground reference for the package  100 . In order to enable a connection between the back side conductor  119  and power supply balls on the circuit side of the package substrate, through-silicon-vias are created in the package substrate. 
     Please refer to  FIG. 5  in conjunction with  FIG. 4  and  FIG. 3 .  FIG. 5  shows a side view of the die  130  in  FIG. 4  assembled into a package  100 , including the through-silicon-vias  117  in the package substrate  115  and the die  130 , as well as the VSS wire bonds  145  connecting the package substrate back side conductor  119  with the die back side conductor  132 . A single via  150  in the die  130  for coupling the bond pad  44  on the circuit side of the die  130  to the back side conductor  132  is shown here for simplicity, and is represented by the vertical dashed lines. A via  117  in the package substrate  115  is also illustrated, and shows how a power supply ball  122  is connected to the package back side conductor  119 . It should be noted that the bond pads on the circuit side of the die  130  still require wire bonds for coupling the bond pads to VDDQ via traces in the package substrate  115 . The use of the vias in the package substrate  115  and the die  130 , however, mean that coupling to VSSQ occurs from front-to-back (circuit side to back side) and therefore the number of wire bonds needed on the circuit side of the die  130  is reduced by approximately one half. 
     As the position and arrangement of bond pads on the circuit side of the die  130 , and traces and power supply balls on the circuit side of the package substrate  115  do not need to be modified, this invention is easy to implement in the industry. Furthermore, when the invention is applied to reducing inductance on the VDDQ power supply rail and more than one common VDDQ supply reference is required, the back plate can be split, thereby allowing the power supply balls to be coupled to numerous different common supply potentials. 
     The through silicon vias in the die can be formed during standard die processing, and the plating of the package substrate and the formation of vias in the package substrate can occur during the package assembly process. Due to the strong common ground reference as a result of the back side plating, when voltage switching occurs, the drop between VSSQ and VDDQ remains constant. Signal Noise Ratio (SNR) is therefore greatly improved. In conventional memory packages, inductance noise may be as much as 400 mV but the present invention can lower the inductance by a magnitude of 10. 
     In summary, the present invention has the double advantages of reducing inductance and increasing the amount of space available for bonding. Utilizing vias rather than wires for VSSQ coupling reduces the amount of inductance and noise, and the backside plating results in a strong common ground reference for the entire package  100 . It is also possible to keep the VSSQ connections as in the prior art and implement the structure of the present invention for achieving a common VDDQ reference. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.