Abstract:
A low impedance power distribution structure and method for substrate packaging of semiconductor chips containing very large scale integrated circuit (VLSI) circuits, such as microprocessors and associated memory, is presented. The power distribution structure incorporates under bump metallurgy (UBM) solder bump forming technology to produce not only solder bump connections that are vertically oriented, but also low impedance distribution wires that are horizontally oriented, and which provide electrical interconnection between various selected electrical contact points, such as solder bumps. These low impedance distribution wires introduce the benefits of low characteristic impedance to the substrate&#39;s power distribution structure.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates generally to semiconductor packaging and chip design, and, more specifically, to a wire structure and construction method for stacked chip modules and packaging of semiconductor chips containing very large scale integrated circuit (VLSI) circuits, such as microprocessors and associated memory. 
     2. Related Art 
     Flip-chip module technology has facilitated increased system density and also increased operating frequency by reducing interconnection distances and increasing signal propagation speed. As chip operating frequencies and power dissipation requirements increase, it has become more difficult to make low impedance power supply connections to a chip or chips. Historically, Controlled Collapse Chip Connection (C4) flip-chip structures have had much better power distribution than wire bond designs, because the C4 chip carrier is typically designed with power supply planes in the chip carrier which can be connected to the chip in many places by C4 solder bumps (usually in a ball grid-array). Wire bonded chips are not as attractive for power distribution because they usually have just peripherally-located pads, and thus power must be distributed within the chip by only the internal chip wiring, which results in higher impedance and increased susceptibility to supply line noise. 
     Recently, stacked chip packages have begun to be used in industry since they allow for high bandwidth interconnects between multiple chips of potentially dissimilar technologies. The goal of moving to System-On-a-Chip (SoC) technologies may actually, in many cases, be less costly to implement as System-On-a-Package (SoP) technology. Flip-chip, or other stacked chip arrangements, offer the best high bandwidth chip-to-chip interconnects, but suffer from power distribution problems similar to the wire bond chip situation described above. 
     Accordingly, there exists a need in the industry for a low impedance power distribution structure, for use in flip-chip type stacked chip modules, which is capable of solving the above-mentioned problems resulting from high impedance wires in power distribution circuits. 
     SUMMARY OF THE INVENTION 
     It is therefore a feature of the present invention to overcome the above shortcomings related to flip-chip type stacked chip module power distribution structures, by providing a method and structure for a low impedance power distribution wire structure. 
     In a first general aspect, the present invention provides a wire structure made from an under bump metallurgy (UBM) process, said wire structure comprising: a substrate having a plurality of first features, said first features including under bump metallurgy; a plurality of second features situated over at least one of said first features, said second features operatively connected to said first features; at least one electrical wire interconnecting said plurality of first features, wherein said electrical wire includes under bump metallurgy, said electrical wire comprising a metal structure having a low impedance and characterized by having substantially the same composition as the contact pads; and wherein said first features and said electrical wire are formed in substantially the same plane. 
     In a second general aspect, the present invention provides an electronic package comprising: a first substrate having a first surface, said first surface including a plurality of first features; a second substrate having a second surface, said second surface including a plurality of second features, wherein said second substrate is positioned substantially parallel to said first substrate, and wherein said second surface is located proximal to and facing said first surface; first electrical wires located on said first surface, said first electrical wires connecting selected ones of said plurality of first features on said first surface; second electrical wires located on said second surface, said second electrical wires connecting selected ones of said second plurality of second features on said second surface; wherein said first substrate and said second substrate are operationally bonded together; and said first electrical wires, said second electrical wires, said first features and said second features are formed with under bump metallurgy (UBM) processing. 
     In a third general aspect, the present invention provides a method of forming an electronic package comprising: providing a first substrate having a first surface, said first surface including a plurality of first features; providing a second substrate having a second surface, said second surface including a plurality of second features, wherein said second substrate is positioned substantially parallel to said first substrate, and wherein said second surface is located proximal to and facing said first surface; providing first electrical wires located on said first surface, said first electrical wires connecting selected ones of said plurality of first features on said first surface; providing second electrical wires located on said second surface, said second electrical wires connecting selected ones of said plurality of second features on said second surface; wherein said first substrate and said second substrate are operationally bonded together; and forming said first electrical wires, said second electrical wires, said first features, and said second features with under bump metallurgy (UBM) processing. 
     The foregoing and other features and advantages of the invention will be apparent from the following more particular description of embodiments of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and inventive aspects of the present invention will become more apparent upon reading the following detailed description, claims and drawings, of which the following is a brief description. 
     FIG. 1 is a perspective view of the surface of a substrate showing a portion of a solder bump grid array modified in accordance with an embodiment of the present invention. 
     FIG. 2 is a plan view of a bottom chip plated solder pattern in accordance with an embodiment of the present invention. 
     FIG. 3A is a plan view of a possible module configuration in accordance with an embodiment of the present invention. 
     FIG. 3B is a cross-sectional view of the module configuration of FIG. 3A taken at sectional line B—B. 
     FIG. 3C is a cross-sectional view of the module configuration of FIG. 3A taken at sectional line C—C. 
     FIG. 4 is a cross-sectional view of an electrical wire in accordance with an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following is a detailed explanation of the structure and method for a low impedance power distribution structure for a flip-chip type stacked chip module, and an electronic package resulting from manufacturing using embodiments of the present invention, with reference to the attached drawings. It should be noted that the same reference numbers are assigned to components having approximately the same functions and structural features in the following explanation and the attached drawings to preclude the necessity for repeated explanation thereof. 
     According to the present invention, a solder bump forming process is utilized to form not only solder bumps, or rounded solder balls, that make chip-to-chip connections, i.e., vertical connections, but also to use the solder bump forming process to make solder lines, i.e., electrical wires, which can provide interconnections between select solder bumps or other contact pads on the same substrate surface, i.e., horizontal connections. These electrical wires serve as very low impedance busses (ie., having sheet rho values of less than a few milli-ohms, depending on line thickness) to supply power from the outer edge contacts for the bottom chip in a stack. The electrical wires further provide improved heat dissipation for the chip, since the electrical wires function as heta sinks for thermal conduction of heat from the chip to the surrounding environment. 
     A typical solder bump forming process utilizes a first masking step to produce a “seed” layer, called the “under bump metallurgy” (UBM) layer. During a typical solder reflow step, the UBM layer prevents diffusion of solder into the underlying chip metallization layers, or so-called barrier corrosion protection. The UBM layer is also used to ensure that there exists good adhesion between the chip metallization layers, and also that there is low contact resistance in the chip metallization layers. 
     In the method of the present invention, the UBM layer is used to not only provide the aforementioned segregation of metals, but is also used to create stand-alone wires. The structure of the present invention includes a UBM layer and solder bump structure wherein the UBM layer consists not only of interconnecting paths, but also of wires. As disclosed herein, known solder bump and UBM processes are used to form a plurality of wires which act as low impedance busses to distribute power within the chip package. These wires or busses may be formed to any desired shape, such as, inter alia, rectilinear wires, curvilinear wires, or various combinations of both. 
     FIG. 1 is a perspective view of the surface  131  of a substrate  100  showing a portion of a solder bump grid array modified in accordance with an embodiment of the present invention. Solder bumps  150  are typical interchip connections, while a plurality of solder bumps  170  are connected by a first low impedance distribution wire  177 . A second low impedance distribution wire  176  is also shown. 
     While a typical chip wire is on the order of about 1 micron tall, typical dimensions for a low impedance distribution wire  177  of the present invention can be about 10 to 150 microns, or more, wide, with a thickness of about 1 to 50 microns, or more. These dimensions illustrate why the low impedance distribution wires of the present invention have a much lower characteristic impedance as compared to the typical known chip wires. 
     Referring now to FIG. 2, a plan view of a bottom chip  200  having a power distribution structure formed from a solder pattern in accordance with an embodiment of the present invention is illustrated. Bottom chip  200  contains a plurality of solder bump connections  205  arranged in a grid array pattern. A first group of solder bumps is connected with low impedance distribution wire  220  to create a power distribution structure having a first voltage potential, such as, inter alia, GND potential. As illustrated in FIG. 2, two additional GND busses  221 ,  222  are similarly formed. 
     As further illustrated in FIG. 2, a second set of low impedance distribution wires  210 ,  211 ,  212  can be formed. This second set of low impedance distribution wires  210 ,  211 ,  212  can be used as a power distribution structure having a second voltage potential, such as, inter alia, VDD. 
     The power distribution structures depicted in FIG. 2 are intended to be exemplary in nature, and are not meant to be taken as limiting. Other configurations for the power distribution structure are possible, and in fact may be better suited for minimizing undesirable impedance characteristics. For example, a power distribution structure utilizing low impedance busses could be formed in which the busses are arranged so that they criss-cross one another in a three-dimensional lattice arrangement. 
     FIG. 3A is a plan view of a possible module configuration in accordance with an embodiment of the present invention. FIG. 3B is a related cross-sectional view of the module configuration of FIG. 3A taken at sectional line B—B. FIG. 3C is also a related cross-sectional view of the module configuration of FIG. 3A taken along sectional line C—C. 
     In FIG. 3A, module  300  includes a chip carrier  315  which has, located around its periphery, a plurality of wire bond connections  320 . Chip carrier  315  further includes an opening  330 , which is typically located about the center region of chip carrier  315 . Opening  330  may be any appropriate shape and size, but is typically rectangular, and sized to accommodate a slave chip. Master chip  310  is emplaced on chip carrier  315 , and appropriate electrical connections are made between chip carrier  315  and master chip  310  through solder bumps  325 . Master chip  310  is also operationally connected to smaller slave chip  340  through a plurality of solder bumps. Slave chip  340  contacts master chip  310  through opening  330  in chip carrier  315 . Low impedance distribution wires for GND potentials  360  and VDD potentials  350  are formed among solder bumps  325  on master chip  310 . 
     The assembly of module  300 , including chips which incorporate low impedance distribution wires of the present invention, may be accomplished using known techniques. For example, the larger or master chip  310  is joined to chip carrier  315  using a conventional solder construction, such as, inter alia, high-melt (i.e., 97/3) solder. In some applications, it may be beneficial to perform quality control testing such as, inter alia, chip testing or burn-in, at this point in the assembly process. The smaller or slave chip  340  is then joined to the master chip  310  by placing it through the opening  330  in chip carrier  315 . The slave chip solder type could be a low-melt solder, a high-melt (i.e., 97/3) solder, or a tin cap. The use of low-melt solders on the master chip  310  can make the bond and assembly procedure very simple because of the solder hierarchy (i.e., the order in which various solders are employed) design. 
     FIG. 4 shows an alternative embodiment of an electrical wire of the present invention. The electrical wire embodiment  400  represents a modified UBM electrical wire  410  on substrate  100 , and further includes a first UBM layer  415  and a second or capping layer  420  which is deposited upon first UBM layer  415 . Capping layer  420 , as a second portion of UBM electrical wire  410 , is suitable for both the standard purposes of UBM, and also for use as wires that carry current laterally across the face of a chip, represented in FIG. 4 by substrate  100 . Capping layer  420  may be formed with any suitable conductive metal or combination of metals, such as, inter alia, nickel, copper, or gold. Capping layer  420  may have from about sub-micron thickness to a thickness of many microns. 
     Embodiments of the present invention have been disclosed. A person of ordinary skill in the art would realize, however, that certain modifications would come within the teachings of this invention. For example, rather than the two chip (master and slave) embodiment as discussed herein, the present invention also encompasses embodiments wherein there are several slave chips for one master chip, or more than two layers of chips, etc. Therefore, the following claims should be studied to determine the true scope and content of the invention.