Patent Publication Number: US-2006003624-A1

Title: Interposer structure and method

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 60/579,415, filed Jun. 14, 2004. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to semiconductor packaging structures and methods.  
     BACKGROUND  
       FIG. 1  is a schematic diagram of a conventional high power semiconductor packaging structure. The current technology for high power semiconductor press pack diodes and thyristors utilizes high tolerance, machined metal plates typically made of Copper or Copper plated Molybdenum. These plates are in tight contact with the power device in order to most efficiently carry the heat and electrical current. As the devices are typically Silicon or Silicon Nitride, they are very brittle and, as such, the surface of the Copper interposer has to be machined very flat in order not to mechanically stress the device. In addition, to maintain good contact, high forces are required between the interposers and the device. This necessitates a massive package casing structure to contain the forces.  
      Another packaging structure and technique is described in U.S. Pat. No. 6,559,561, which is incorporated by reference in its entirety, as though fully set forth herein. That patent describes a process including first weaving a plurality of electrically non-conductive strands (e.g., fiberglass yarns) and at least one electrically conductive strand (e.g., a copper wire) to form a woven fabric. Upper and lower surfaces of the woven fabric thus formed are exposed. Next, the woven fabric is impregnated with a resin material to form an impregnated fabric and, thereafter, the impregnated fabric is cured to form a cured fabric. The upper and lower surfaces of the cured fabric are then planed. The planing of these surfaces segments the at least one electrically conductive strand and forms a PCB substrate.  
      An improved packaging structure is desired. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram of a conventional packaging structure.  
       FIG. 2  is an isometric view of an exemplary structure according to an embodiment of the invention.  
       FIG. 3  shows the structure of  FIG. 2  being used to provide power to and remove heat from a device.  
       FIGS. 4A and 4B  shows application of conductive plates to the structure of  FIG. 2 , for providing power more uniformly and removing heat more uniformly.  
       FIG. 5  shows another embodiment of a package.  
       FIGS. 6A-6I  show steps of fabricating the structure of  FIG. 5 . 
    
    
     DETAILED DESCRIPTION  
      U.S. Provisional Patent Application No. 60/579,415, filed Jun. 14, 2004 is incorporated by reference herein in its entirety as though fully set forth below.  
      This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,”etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise.  
      A structure and application of materials is disclosed herein, using a composite weaving technology that can separate thermal management from electronic power management.  
       FIG. 2  shows an exemplary embodiment that separates thermal management from electronic power management. The structure  200  has a core of a compliant material  210  with a high thermal conductivity, and relatively low electrical conductivity. For example, graphite fibers, rovings, strands or yarn  210  may be used. Examples of alternative materials that could be substituted for graphite include but are not limited to aluminum Nitride, Silicon Carbide, Intrinsically Conductive Polymer. Pitch based graphite yarn has very high thermal conductivity and can be utilized for heat transfer. In some embodiments, a high thermal conductivity graphite of about 800 W/mK is used. The thermal and electrical properties of the material  210  are determined by the manufacturing process used. For example, in some embodiments, thermal conductivity may range from 1000 W/mK (Watts per meter Kelvin) to 8.5 W/mK with corresponding electrical resistivity of 1.3 mico ohms centimeters to 18 micro ohm centimeters. The total thickness of the thermally conductive core material  210  may vary, depending on the die thickness. For example, the graphite should have a minimum thickness approximately equal to the thickness of the semiconductor die and a maximum thickness of about 20 times the die thickness. As the amount of heat generated is directly proportional to the size of the die, the larger the die, the thicker the graphite required.  
      The layers of thermally conductive core yarns  210  may be individually woven layers or the fibers within an individual layer may not be woven to each other (except by the wire  220 ). In some embodiments, a plurality of layers of aligned graphite yarns may be provided, with alternating parallel planar layers oriented in orthogonal (X and Y) directions from each other.  
      The example of  FIG. 2  shows three layers of graphite yarn  210 , but any desired number of layers may be used. By running yarn in different layers in both X and Y directions, heat transfer in both directions is ensured without relying on extensive transverse heat transfer between adjacent parallel yarns.  
      Although  FIG. 2  shows yarn layers that are not individually woven (except by the wire) to each other, alternatively, one or more individually woven layers of graphite fibers or yarns may be provided. These individually woven layers are then woven to each other by the wire  220 .  
      A plurality of conductive, both insulated and/or non-insulated, (e.g., metal, such as copper) wires  220  are woven through the thermally conductive core layer  210 . An example of a suitable conductor is copper having a resistivity of about 1.74 μohm-cm. Any weaving technique may be used, including but not limited to conventional weaving techniques. This weaving provides a plurality of insulated and/or non-insulated wires  220  extending in the Z direction, orthogonal to the plane of the thermally conductive core layers  210 . If the thermally conductive core material  210  is woven, the insulated and/or non-insulated conductive wire  220  may replace strands in the weaving technique used, or the conductive strands may be in addition to the conventional weave. With the insulated and/or non-insulated conductive wire  220  woven into the material, the electrical power can flow from one side of the interposer  200  to the other (parallel to the Z axis). Although the exemplary wire material is copper, other insulated and/or non-insulated conductive materials, may be used such as, but not limited to, gold wire, aluminum wire, an electrically conductive polymer wire or a combination thereof.  
      With the wire  220  extending in the Z direction, the wires can contact the various fibers, strands or yarns  210  at several points along each fiber, strand or yarn, to conduct heat directly to the thermally conductive strands.  
      The diameter of the electrically insulated and/or non-insulated conductive wire  220  depends on the thickness of the structure  200  and the desired density of electrically conductive vias disposed therein. For example, the wire diameter may be between about 10 microns and about 500 microns and is preferably between about 15 microns and about 200 microns.  
      In some embodiments, one or more additional insulating layers  230  are provided on both sides of the core layers  210  for electrical isolation. For example,  FIG. 2  shows a single layer of insulating fibers, rovings, strands or yarns  230  adjacent to each major face of the core thermally insulating layer  210 . E-glass may be used for electrical isolation in some embodiments. Other examples of materials for the optional insulating layers  230  may include, for example, fiberglass, S-glass, polyester or other polymers, tetrafluoroethylene, “KEVLAR®”, Type 1064 Multi-End Roving and Hybon 2022 Roving available from PPG Industries. In other embodiments (not shown in  FIG. 2 ) the insulating layers  230  may be omitted.  
       FIG. 3  shows a configuration in which the structure  200  described above is incorporated into a package for power and thermal management. A device  300  to be cooled and supplied with power is interfaced to one major face of the structure  200  of  FIG. 2 , and a pressure plate  305  is interfaced to the other major face. A metal matrix  310  is placed on each side of the structure, and a heat sink  320  is interfaced to the metal matrix. The metal-metal matrix  310  acts as a secondary heat sink of lower cost and/or higher mechanical stability than the graphite. These heat sinks  320  can be made of materials such as aluminum, aluminum/silicon carbide, copper, copper-tungsten, copper-molybdenum, aluminum-aluminum-nitride, for example. Although  FIG. 3  only shows the metal matrix  310  and heat sink  320  on two sides of the structure  200 , in other embodiments, the metal matrix and heat sink may be on three or more sides of the structure  200 .  
      As shown in  FIG. 3 , by weaving the thermally conductive core material  210  (e.g., graphite) into the material and attaching the ends to a heat sink  320 , the heat generated by the device can be flowed to the outside edges of the package (parallel to the X and Y axes), while allowing the electrical power to flow in the Z direction, through the thickness of the structure  200 . Additionally, as stresses are built up due to thermal gradients and mismatches, the capability of fabric  210  to move in the bias direction allows the relief of these thermal stresses. The structure  200  shown in  FIG. 2  is more capable of moving in the bias direction to relieve thermal stress than conventional structures such as that shown in  FIG. 1 .  
      Properties:  
      Thermal management is separated from electrical management by using a thermally conductive, electrically insulating material  210 , such as graphite fibers.  
      Electrical management is separated from thermal management by using insulated and/or non-insulated conductive wire material  220 .  
      Coefficient of thermal expansion mismatches are handled by the fact that woven material  210  is compliant in the bias direction and can yield to thermal stresses.  
      The accuracy of assembly is not required to be as critical for surface contact as prior art technology, because the contact points can “float”. For example, if a fiber, roving or yarn  210  moves longitudinally relative to one of the vertical portions of the wire  220 , the fiber, roving or yarn  210  can still contact the wire  220  at a different point along the length of the fiber, roving or yarn  210 .  
      There is no need to impregnate the structure  200  with any resin or adhesive, simplifying fabrication, and eliminating a curing step. Also, the absence of an impregnating resin or adhesive enhances the compliance and ability to accommodate coefficient of thermal expansion mismatches.  
       FIGS. 4A and 4B  show application of a soldered plate  400  to the structure  200 . The plate  400  may be formed of a highly conductive material, such as copper, for spreading heat and power across the length and width of the package. The plate  400  spreads the electrical power among the woven copper conductors  220 .  
       FIG. 5  shows another example, showing that it is also possible to fabricate circuitry on a non-resin impregnated substrate. Fabrication of the structure  500  begins with the structure  200  of  FIG. 2 . In the structure  500  of  FIG. 5 , the wires  220  are singulated to create vias  520 . One can use laser, chemical etching, or mechanical cutting during the weaving operation, by utilizing a wire loom or other cutting means. After Aluminum Nitride  540  (or other suitable dielectric is plasma deposited the circuitry fabrication would be similar to current PCB practices. Several deposition processes may be used. For example, CVD (Chemical Vapor Deposition), Plasma arc spray, HVOF (High Velocity Oxygen Fueled) can be used depending on the material to be deposited Alternatives to the Aluminum Nitride  540  include materials such as Polyamide, Silicon Dioxide, Aluminum Oxide, Glass Silica, Liquid crystal polymers  
       FIGS. 6A-6I  show a process flow for this method.  
      In  FIG. 6A , the structure  200  of  FIG. 2  is fabricated.  
       FIG. 6B  shows the structure after singulation of the wires  220  to form vias  520 .  
       FIG. 6C  shows the structure after plasma deposition of aluminum nitride (or other) dielectric.  
       FIG. 6D  shows the structure after the vias  520  have been exposed, for example by laser etching.  
      In  FIG. 6E , the entire surface is coated with a layer of metal (e.g., copper). The metal is then coated with a dielectric, such as a resin laminate.  
      In  FIG. 6F , a photoresist is applied over the copper.  
      In  FIG. 6G , the photoresist is selectively etched to expose the vias  520 .  
      In  FIG. 6H , the photoresist is removed, leaving the dielectric layer with exposed vias therebeneath.  
      In  FIG. 6I , circuit patterns are formed over the dielectric, using any suitable deposition technique.  
      The structure  500  is useful, for example, for packaging Insulated Bipolar Gate Transistor (IBGT), because the same thermal and power problems exist as the diodes and thyristors but circuitry is also required.  FIGS. 5 and 6 I show an example with circuitry on the top surface (Similarly, circuitry on the bottom could be handled in the same manner.  
     Summary of the Exemplary Embodiments  
      1. Some embodiments include a structure comprising: 
          (a) at least one layer of thermally conductive, electrically insulating fibers, rovings, strands or yarns having first and second major surfaces; and     (b) at least one electrically insulated and/or non-insulated conductive wire or strand woven with the thermally conductive fibers, rovings, strands or yarns so that the electrically insulated and/or non-insulated conductive wire or strand extends from the first major surface to the second major surface in a plurality of locations.        

      2. In some embodiments, the thermally conductive, electrically insulating fibers, rovings, strands or yarns comprise graphite.  
      3. Some embodiments have the thermally conductive, electrically insulating fibers, rovings, strands or yarns oriented in two directions that are perpendicular to each other.  
      4. In some embodiments, the at least one electrically insulated and/or non-insulated conductive wire or strand comprises one of the group consisting of copper, gold wire, aluminum wire, an electrically insulated and/or non-insulated conductive polymer wire or a combination thereof.  
      5. Some embodiments further comprise at least one layer of insulating fibers, rovings, strands or yarns facing a major surface of the layer of thermally conductive, electrically insulating fibers, rovings, strands or yarns, and woven thereto by the electrically insulated and/or non-insulated conductive wire or strand.  
      6. In some embodiments, the structure is interposed between a device and a pressure plate without impregnating the structure.  
      7. In some embodiments, the thermally conductive, electrically insulating fibers, rovings, strands or yarns are thermally coupled to a heat sink.  
      8. In some embodiments, a metal plate is joined to the electrically conductive wire or strand on at least one of the major surfaces.  
      9. In some embodiments, the electrically insulated and/or non-insulated conductive wire or strand is cut to form a plurality of vias.  
      10. Some embodiments further include a layer of dielectric disposed over the conductive wire or strand, and at least one printed circuit path formed over the layer of dielectric.  
      11. Some embodiments include a method comprising: 
          (a) providing at least one layer of thermally conductive, electrically insulating fibers, rovings, strands or yarns having first and second major surfaces; and     (b) weaving at least one electrically insulated and/or non-insulated conductive wire or strand with the thermally conductive fibers, rovings, strands or yarns so that the electrically insulated and/or non-insulated conductive wire or strand extends from the first major surface to the second major surface in a plurality of locations.        

      12. In some embodiments, the thermally conductive, electrically insulating fibers, rovings, strands or yarns comprise graphite.  
      13. In some embodiments the method includes orienting the thermally conductive, electrically insulating fibers, rovings, strands or yarns in two directions that are perpendicular to each other.  
      14. In some embodiments, the at least one electrically insulated and/or non-insulated conductive wire or strand comprises one of the group consisting of copper, gold wire, aluminum wire, an electrically insulated and/or non-insulated conductive polymer wire or a combination thereof.  
      15. Some embodiments further comprise weaving at least one layer of insulating fibers, rovings, strands or yarns onto a major surface of the layer of thermally conductive, electrically insulating fibers, rovings, strands or yarns, with the electrically conductive wire or strand.  
      16. Some embodiments include interposing the structure between a device and a pressure plate, for supplying power to and removing heat from the device.  
      17. Some embodiments include thermally coupling the thermally conductive, electrically insulating fibers, rovings, strands or yarns to a heat sink.  
      18. Some embodiments include joining a metal plate to the electrically insulated and/or non-insulated conductive wire or strand on at least one of the major surfaces.  
      19. Some embodiments include cutting the electrically insulated and/or non-insulated conductive wire or strand to form a plurality of vias.  
      20. Some embodiments further include forming a layer of dielectric over the insulated and/or non-insulated conductive wire or strand, and forming at least one printed circuit path over the layer of dielectric.  
      Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the invention should be construed broadly, to include other variants and embodiments, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.