Abstract:
Wafer-level chip-scale packaging technology is used for improving performance or reducing size of integrated circuits by using metallization of pad-to-bump-out beams as part of the integrated circuit structure. Chip-scale packaging under bump metal is routed to increase the thickness of top metal of the integrated circuit, increasing current carrying capability and reducing resistance. An exemplary embodiment for a power MOSFET array integrated structure is described.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     REFERENCE TO AN APPENDIX 
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     BACKGROUND 
     1. Technical Field 
     This disclosure relates generally to integrated circuits and, more particularly to integration of chip-scale packaging input-output bump-connection metallurgy into integrated circuit structures. 
     2. Description of Related Art 
     Semiconductor integrated circuits (“IC”) in the state of the art have been able to pack millions of circuit elements into a relatively small die, or “chip”, e.g., having lateral area footprint, e.g., a ¼″ by ¼″. Most ICs are designed with input-output (“I/O”) pads located along the periphery of the chip; some requiring hundreds of such pads. These pads are then wire-bonded to connect the IC to the macro-world of a printed wire board (“PWB”), also known as printed circuit board (“PCB”), and surrounding discrete elements and other IC electronics on the board. This conventional perimeter-lead surface mount technology (“SMT”) for complex circuitry with appropriate interconnects often requires a chip carrier several times greater in size than the chip itself. 
     For mobile appliances—e.g., cellular telecommunications products, portable digital assistants (“PDA”), notebook computers, and the like—or applications where physical space for computers and instrumentations is extremely valuable—e.g., aircraft, space shuttles, and the like—individual component size and weight are factors which are critical to successful design. Thus, there is a conflict between a higher density of IC elements on the chip with attendant higher input/output (“I/O”) needs and a simultaneous demands for continuing miniaturization with increased functionality. 
     Wafer-level packaging (“WLP”), wherein a single IC die and its mounting package are manufactured and tested on a multi-die wafer produced by the IC manufacturer prior to singulation into individual chips, offers many advantages to the chip manufacturer. One WLP solution known in the art is generally referred to in the art as chip-scale packages (“CSP”). Chip-scale packaging technology, where the peripheral pad configuration is redistributed, provides die-sized packaging, allowing more condensed PCB patterns, also referred to in the art as “land patterns” where elements have a specific area “footprint.” 
     Exemplary, conventional, chip-scale technology is demonstrated by  FIG. 1A and 1B , taken from Semiconductor International magazine, Oct. 2000, pp. 119-128, “Wafer-Level Packaging Has Arrived,” by Dr. Philip Garrou, illustrating the process  100 ,  FIG. 1A , and resultant structure  102 ,  FIG. 1B , for chip-scale packaging I/O redistribution. As shown in  FIG. 1A , IC  101  peripheral I/O pads  103  are redistributed to bumps  107  via known manner processes, including a “Metallization” step  105  from pads  103 . The process continues to an I/O bump formation step wherein the bumps  107  are located inwardly from the chip periphery. Conductive material (such as a metal, e.g., copper) beams  109  are lithographically defined superjacent the chip passivation layer  111  (e.g., a plasma nitride or the like) and within a protective-covering-stress-absorbing material (e.g., resin, polyimide, or the like)  113 , providing a conventional wisdom IC, “Silicon,” with chip-scale packaging structure as shown in  FIG. 1B. A  variety of implementations are described by Garrou. In current wafer-level packaging, these additional layers of the chip-scale package are generally are so formed on the wafer after the die fabrication is completed, yielding a plurality of packaged die on the wafer, which has many advantages for the manufacturer. A thereafter singulated die with chip-scale package  115  with eight bumps  107  is illustrated in  FIG. 1C , showing that the total footprint is essentially the same as the die area. The present invention relates to further discoveries in this regard. 
     Many publications describe the details of common techniques used in the fabrication of integrated circuits that can be generally employed in the fabrication of complex, three-dimensional, IC structures; see e.g.,  Silicon Processes , Vol. 1-3, copyright 1995, Lattice Press, Lattice Semiconductor Corporation (assignee herein), Hillsboro, Oreg. Moreover, the individual steps of such a process can be performed using commercially available IC fabrication machines. The use of such machines and common fabrication step techniques will be referred to hereinafter as simply: “in a known manner.” As specifically helpful to an understanding of the present invention, approximate technical data are disclosed herein based upon current technology; future developments in this art may call for appropriate adjustments as would be apparent to one skilled in the art. 
     BRIEF SUMMARY 
     The basic aspects of the invention generally provide for use of chip-scale packaging metallization as part of an integrated circuit active element metallization layer. In an exemplary embodiment, the present invention provides for power MOSFET (metal-oxide-semiconductor-field-effect-transistor) size reduction by including the use of chip-scale metallization as part of the die structure itself. 
     In aspect of the invention, an exemplary embodiment is shown as an integrated circuit structure including chip-scale packaging, the structure including: a plurality of active elements in a surface of a semiconductor die; at least one conductive-material bus electrically interconnecting said active elements; said chip-scale packaging including at least one, conductive-material, input-output bump extending outwardly from said die for electrically connecting said plurality of active elements to off-die electronics, and a beam of conductive material connecting said bus to said bump; and said bus having a construction wherein the conductive material forming said beam is extended to regions of said structure for thickening of said bus such that resistance of said bus is reduced. 
     In another aspect of the invention, an exemplary embodiment is shown as a power MOSFET array integrated circuit device including: at least a first row of drain regions in a semiconductor surface; at least a second row of source regions in said surface; channel regions in said surface, separating source regions of said second row from respective drain regions of said first row; a gate structure superjacent respective said channel regions; a first conductive trace for electrically coupling said drain regions to a first input-output pad; a second conductive trace for electrically coupling said source regions to a second input-output pad; a first conductive beam for electrically coupling said first input-output pad to a first input-output chip-scale packaging bump; and a second conductive beam for electrically coupling said first input-output pad to a second input-output chip-scale packaging bump, wherein conductive material forming said first conductive beam is routed onto and thickens said first conductive trace for reducing resistance thereof, and conductive material forming said second conductive beam is routed onto and thickens said second conductive trace for reducing resistance thereof. 
     It is another aspect of the present invention to provide a method for improving (R ON )*Area figure-of-merit for an integrated circuit, including a top metal layer, where R ON  is a predetermined resistance characteristic and Area is the lateral footprint area of the integrated circuit, the method including: forming active elements, including said top metal layer, in and on a first surface a semiconductor substrate; and forming metal beams for chip-scale packaging input-out bumps such that metallization of said beams also extends onto said top metal layer, increasing the thickness thereof. 
     Another aspect of the present invention provides an integrated circuit die having an array of MOSFET devices, each having at and interconnect traces, having individual elements of said devices sharing a common top metal trace and pad respectively, the die further including bump out contacts with metal beams for connecting to said MOSFET elements respectively, the die further comprising: said top metal trace in contact over a top surface thereof with a respective said one of said metal beams formed in either a long, narrow, single strip via juxtaposed with the IC top metal, a first via connecting the metal down to the top metal as the bump-out metal comes into the IC device active element regions which extends across the active element regions to a second via at a distal end or, wherein the MOSFET is an array broken up into two or more sections having a plurality more tack down vias therefor. 
     The foregoing summary is not intended to be inclusive of all aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Brief Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprise the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  (Conventional) is a schematic chip-scale process flow diagram. 
         FIG. 1B  (Conventional) is a partial cross section, elevation view, of a chip-scale I/O redistribution die formed in accordance with the process as shown in FIG.  1 A. 
         FIG. 1C  (Conventional) is a schematic perspective view of a singulated wafer-level chip-scale package and attached die resultant from a process and fabrication as shown in  FIGS. 1A and 1B . 
         FIG. 2A  in accordance with a first exemplary embodiment of the present invention is a schematic IC layout view for a simplified power MOSFET array. 
         FIG. 2B  is an elevation view for a partial cross section of the structure as shown in FIG.  2 A. 
     
    
    
     Like reference designations represent like features throughout the drawings. The drawings in this specification should be understood as not being drawn to scale unless specifically annotated as such. 
     DETAILED DESCRIPTION 
     To simplify the description of the present invention, a simplified embodiment of an IC structure comprising an array of power MOSFETs is used. No limitation on the scope of the invention is intended by the inventors in using this simple device example, nor should any be implied therefrom. Those skilled in the art will recognize that the basic methodology of this described technology can be extended to most types of die, having other active elements besides MOSFET elements—e.g., emitter and collector pairs of bipolar transistors, anode/cathode pairs, diode poles, programmable logic arrays elements, and the like—which are amenable to chip-scale packaging and where a PIE characteristic is definable. 
     As set forth in the Background section hereinabove, miniaturization of ICs is a continuing goal; chip-scale packaging is a technology in furtherance of this goal. One measure-of-performance, or figure-of-merit, for power MOSFET ICs, having a given operating voltage, is to have a lowest possible “(R ON )*Area” product in milliohms-mils 2 . That figure-of-merit is also referred to hereinafter as PowerFET Interconnect Efficiency (“PIE”). However, it is recognized that for power MOSFET chip designs there is a true PIE characteristic reflecting the difference between an ideal test pattern, defining 100% efficiency, and losses induced inherently by silicon implementation and packaging result in a PIE in the approximate range of 50%-75%. Much of the loss in efficiency may be attributable to the metal electrical traces running to I/O pads and particularly to the wire-bonds used in SMT packaging. The use of CSP packaging therefore eliminates the wire-bonds provides one improvement in the PIE characteristic by decreasing the R ON  factor for a given chip footprint. Simply increasing each buses thickness during die fabrication to reduce R ON  itself for a given chip footprint is complex, e.g., requiring added steps such as electro-less plating. The present invention improves the state of the art by using CSP technology to lower the PIE characteristic for a chip, and further, where desirable for miniaturization, to reduce the footprint for a given PIE value. 
       FIG. 2A , a partial device layout view, schematically illustrates a simplified MOSFET array IC device  201  comprising standard power MOSFETs  203 , having respective source  205 , “S,” and drain  207 , “D,” regions. A gate structure  209  overlays the channel region between each source  205  and drain  207  of each MOSFET  203 . Contacts  211   S ,  211   D  to each respective source  205  and drain  207  are provided for connecting metal traces  212   S ,  212   D  to I/O pads  213   S ,  213   D  for the respective source/drain regions. It will be recognized that in many implementations, the sources  205  are a continuous strip and the drains  207  are a continuous strip. Note here, that in alternative IC embodiments, these source/drain metal traces may be, in fact, any top metal layer—commonly referred to in the art as “Metal 1,” “Metal 2,” “Metal 3,” et seq., depending on the specific IC functionality and I/O requirements of particular elements of the chip&#39;s active devices. 
     Turning also to  FIG. 2B , a schematic elevation view of device  201  through plane A—A of  FIG. 2A  is provided. The illustration is of a cross-section through separate drain regions, but again, in other implementations, those regions may be a continuous strip. In terms of the prior art, it is known that a passivation material normally overlays the metal traces  212   S,D . Looking also back to  FIG. 1B , it should be recognized that the pad  103  there is equivalent to a pad  213   S,D  in accordance with the present invention as shown in  FIGS. 2A and 2B . Conventionally, next, a chip-scale metal beam  109  used for the pad-to-bump redistribution to I/O bump  107  of  FIG. 1B  would be formed so as to be embedded in the protective-covering-stress-absorbing material  113 . Another via would be required above the pad  103  to bring the metal beam  109  down to the pad  103 . 
     Now however, referring back to  FIGS. 2A and 2B , in order to improve PIE in accordance with the present invention, in redistribution of an I/O pad  213   S,D  to a bump  107  (pseudo-isometrically shown in FIG.  2 B), metal traces  212   S,D  are opened, respectively, to Via 2  by appropriate masking and etching, or other in a known manner, in a geometry wherein the formation of the beam  109  also deposits metal  109 ′ on the associated trace. Remember Via 2  was conventionally used only for the pad-to-bump interconnect as shown in  FIG. 1B , but now Via 2  for each respective region is also open, forming a slot or trench, across the top surface of the already formed metal trace  212   S,D  comprising respective source and drain interconnects. Therefore, deposition, or reflow, of the heretofore CSP metal—as in step  105 , “Metallization,” of the Redistribution Process Flow of FIG.  1 A—to form what in  FIG. 1B  is only an encapsulated beam  109  from the I/O pad  103  out to the bump  107 , now also lays the same metal  109 ′,  FIG. 2B , superjacently to the source/drain metal  212   S,D . In this manner, the first metallization of the I/O redistribution process is combined with the conventional Pad Mask step of die fabrication. 
     In other words, the masking and metallization steps are modified so that rather than merely being formation of the redistribution beam  109  from a pad  213  to an associated bump  107 , the metal  109 ,  109 ′ also forms superjacent an exposed surface of the top metal  212  of the device as shown in FIG.  2 B. This effectively increases the thickness of the metal traces  212   S,D . Increasing the thickness of the metal traces reduces the resistance, therefore enhancing the electrical current capability, and therefore lowers PIE for the same die area for a given IC operating voltage. Depending on the specific implementation and IC design in accordance with the present invention, it has been determined that the (R ON )*Area product may be improved by a factor in the range of approximately 10-30%. 
     There are at least three fundamental fabrication process techniques for having the bump-out metal  109 ,  109 ′ also be in direct contact with the IC top metal  212  to achieve this structure. First, a long, narrow, single strip via juxtaposed with the IC top metal may be provided. Second, a first Via connecting the metal down to the top metal as the bump-out metal comes into the IC device active element regions, which then stretches across the active element regions to a second via at a distal end. Third, similar to the second, except wherein the device transistor array is broken up into two or more sections to allow more tack down vias. The first is preferred where the element array is wide enough to support a single, long, narrow via. Thus, it should be recognized by those skilled in the art that a variety of implementations may be constructed in accordance with the need of any specific IC design. 
     Moreover, looking a complementary aspect of the present invention, if the current R ON  is an acceptable operational design specification, conventional die shrink technology may be employed to reduce the die footprint. In other words, for a predetermined specification for R ON , lateral footprint area of said structure may be reduced by a factor in the range of approximately 10-30%. 
     The possibility of modifications and variations for other types of integrated circuits, discrete devices, logic devices, thin-film resistor arrays, and the like, will be apparent to practitioners skilled in the art. Clearly, a variety of specific geometric arrangements for the beams and overlay of beam metal onto the top metal can be tailored for each implementation. The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements during the term of the patent, and that adaptations in the future may take into consideration those advancements, in other word adaptations in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . .”