Patent Publication Number: US-2005127484-A1

Title: Die extender for protecting an integrated circuit die on a flip chip package

Description:
FIELD OF THE INVENTION  
      This application relates generally to integrated circuit (IC) packaging and in particular to a die extender for protecting an IC die on a flip chip package during handling.  
     BACKGROUND OF THE INVENTION  
      Integrated circuits are fabricated on the surface of a semiconductor wafer in layers and later singulated into individual semiconductor devices, or “dies.” Many fabrication processes are repeated numerous times, constructing layer after layer until fabrication is complete. Metal layers, which typically increase in number as device complexity increases, include patterns of conductive material that are vertically insulated from one another by alternating layers of insulating material. Conductive traces are also separated within each layer by an insulating, or dielectric, material. Vertical, conductive tunnels called “vias” typically pass through insulating layers to form conductive pathways between adjacent conductive patterns. Since the material of a semiconductor wafer—commonly silicon—tends to be relatively fragile and brittle, dies are often assembled into protective housings, or “packages,” before they are interconnected with a printed circuit board (PCB).  
      Flip-chip interconnect technology supports “area array interconnection,” a technology in which the die (or “chip”) can be mechanically and electrically connected to a substrate or board through an array of solder bumps on the active face of the die. As the entire active face of the die (and not just the periphery) can be used for interconnections, this technique increases the number of connections that can be made for a given die size. The die is affixed to the substrate facedown (or “flipped”) by slightly melting the solder bumps in an oven reflow process, attaching them to the substrate. The solder bump area is often reinforced by introducing an epoxy underfill between the die and the substrate in order to improve solder joint reliability. Electrical performance can also be improved by reducing inductance and capacitance, as a result of the reduced distance between the active surface of the die and the underlying board over non-flip-chip configurations.  
      In one form of flip chip package a lid is attached to protect the backside of the die.  FIG. 1  illustrates a perspective view of an example of a semiconductor package  10  of the prior art. The package  10  includes the substrate  12 , the IC die  14  attached in a flip chip position, and a lid  16  attached with a lid adhesive  18 . The lid  16  acts as a protective covering for the die  14 , providing mechanical stability and permitting easier handling of the package  10 . Several issues are associated with employment of a lid  16  in a flip chip package  10 . A heat sink is typically applied to the lid  16  in order to aid cooling of the IC die  14 . The lid  16  limits heat transfer from the die  14  to the heat sink. The lid  16  is also expensive and the process of attaching the lid requires extra processing and temperature cycling steps. In addition, when the package  10  is put through a temp cycle (TC) stress test, the adhesives employed to hold the lid  16  in combination with the rigidity of the lid  16  cause stresses on the die  14 , which reduce the die&#39;s  14  reliability.  
      In another form of flip chip package, the package does not have a lid, or is lidless. In the case of a lidless package, a heat sink may be applied directly to the surface of the die  210 . The heat sink may contact the die  210  such that the surface of the heat sink is not flush or perfectly planar with the surface of the die  210 . Such uneven contact may cause cracks in the IC  210  or damage the edge of the die  210  if the heat sink makes inexact contact with either the surface of the die  210  or a die  210  edge. Such damage may harm the integrity of the electrical connections of the package  200 . Thus, a need exists for an improved flip chip package that reduces stresses on and risk of physical damage to the IC die, while improving heat transfer during heat sinking.  
     SUMMARY OF THE INVENTION  
      In an embodiment, a semiconductor package is provided including a substrate, an integrated circuit die, and a die extender disposed on the substrate around the die. In another embodiment, the die extender protects the die from damage. In another embodiment, the die extender frames the die. In another embodiment, the die extender is at least as thick as the die. In another embodiment, the die extender is of a thickness such that a heat sink device applied to the package contacts the die extender prior to contacting the die. In an embodiment, the die extender is in contact with the die. In another embodiment, the die extender is not in contact with the die.  
      In an embodiment, a method is provided including attaching an IC die to a substrate and disposing a die extender on the substrate around the perimeter of the die wherein the die extender protects the die. In another embodiment, the method provided includes applying a heat sink compound or a metal layer to the die wherein the heat sink compound or metal layer further protects the die and provides thermal contact between the die and a heat sink. In another embodiment, the heat sink compound or metal layer may be applied via sintering, sputtering, laser processing, or placing of a pre-formed piece.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a cross-sectional view of a flip chip package of the prior art.  
       FIG. 2  illustrates a cross-sectional view of an embodiment of a lidless flip chip package including a die extender wherein the die extender is planar with the surface of the die.  
       FIG. 3  illustrates a cross-sectional view of an embodiment of a lidless flip chip package including a die extender comprised of two components wherein one component acts as an adhesive.  
       FIG. 4  illustrates a cross-sectional view of an embodiment of a lidless flip chip package including a die extender comprised of two components wherein one component is inserted in the second component.  
       FIG. 5  illustrates a cross-sectional view of an embodiment of a lidless flip chip package including a die extender wherein the die extender is not planar with the surface of the die.  
    
    
     NOTATION AND NOMENCLATURE  
      Certain terms are used throughout the following description and claims to refer to particular.system components. As one skilled in the art will appreciate, companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”.  
      The term “integrated circuit” or “IC” refers to a set of electronic components and their interconnections (internal electrical circuit elements, collectively) that are patterned on the surface of a microchip. The term “semiconductor device” refers generically to an integrated circuit (IC). The term “die” (“dies” for plural) refers generically to an integrated circuit, in various stages of completion, including the underlying semiconductor substrate and all circuitry patterned thereon. The term “flip chip” refers to a bumped die, which is designed for a face-down direct interconnection with an underlying electrical component through a C 4  or bumped connection.  
      The terms “semiconductor package,” “integrated circuit package,” “IC package,” or “package” refer generically to a die mounted within a package, as well as all package constituent components. To the extent that any term is not specially defined in this specification, the intent is that the term is to be given its plain and ordinary meaning.  
     DETAILED DESCRIPTION OF EMBODIMENTS  
       FIG. 2  illustrates an embodiment of a semiconductor package  200  incorporating a die extender  205 . The die extender  205  frames an integrated circuit die  210  and, like the die  210 , the die extender  205  is affixed to the package substrate  215 . For purposes of describing  FIGS. 2 through 5 , unless otherwise stated, the “surface” or “top surface” of the die  210  or die extender  205  refers to the surface opposite the substrate  215 . In addition, the terms “thick” or “thickness” when used to describe the die  210  or die extender  205  refer to the distance from the surface of the die  210  or die extender  205  in contact with the substrate  215 , to the surface of the die  210  or die extender  205  opposite the substrate  215 . Further, the terms “frames” or “framing,” when used to describe the position of the die extender  205  relative to the die  210 , may refer to a continuous or intermittent die extender  205 , and may refer to a die extender  205  that is or is not in contact with the die. In an embodiment, the IC package  200  is a flip chip package and the die  210  is attached in a flip chip position. In another embodiment, the package  200  is lidless. In another embodiment, the top surface of the die extender  205  is substantially planar with the top surface of the die  210 . In another embodiment, the die extender  205  is at least as thick as the die  210 . In another embodiment, the die extender  205  is thicker than the die  210 .  
      By framing the die  210  and effectively extending the top surface of the die  210 , the die extender  205  reduces the risk of damage caused by operation of a heat sink on the die  210 , and acts as a shield for and relieves stresses on the die  210 . In an embodiment, the die extender acts as a protective ring around the die. In another embodiment, the outermost perimeter of the die extender  205  is substantially aligned with the perimeter of the substrate  215 . In another embodiment, the outermost perimeter of the die extender  205  is within the outer perimeter of the substrate  215 . In another embodiment, the die extender  205  is in contact with the die  210 . In another embodiment, the innermost perimeter of the die extender  205  contacts and conforms to the shape of the outside perimeter of the die  210 . In another embodiment, the die extender  205  is not in contact with the die  210 .  
      The die extender  205  generally occupies some or all of the surface area of the substrate  215  that is not occupied by the die  210 . In the embodiment illustrated by  FIG. 2 , the die extender  205  is made up of a continuous formation on the surface of the substrate  215  that frames the die  210 . In another embodiment, the die extender  205  includes two or more consistently or intermittently spaced formations on the surface of the substrate  215 . As an example, the die extender may be a continuous formation that frames the die and possesses a footprint or breadth such that it occupies only a small percentage of the surface area of the substrate, forming a relatively thin protective ring around the die. In another example, the die extender may be a series of consistently or intermittently spaced columns of a variety of breadths and geometries that occupy the surface of the substrate.  
       FIG. 3  illustrates a cross-sectional view of an embodiment of a semiconductor package  300  including a die extender  350 . In this embodiment, the die extender  350  is a protective ring that frames the IC die  310  and occupies most of the surface area of the substrate  320  not occupied by the die  310 . The die extender  350  is not in contact with the IC die  310 , as evidenced by the gap  360  shown between the die extender  350  and the die  310 . In an embodiment, the die extender  350  is made up of two or more components. In another embodiment, the die extender  350  includes a first layer  330  disposed on the substrate  320  and a second layer  340  disposed on the first layer  330 , whereby the first layer  330  attaches the second layer  340  to the substrate  320 . In another embodiment, the first layer  330  comprises an adhesive or glue.  
      The surface of the second layer  340  opposite the first layer  330  may be substantially planar with the top surface of the die  310 , or may rise above the top surface of the die  310  such that a heat sink (not shown) would first contact the surface of the second layer  340  before making contact with the die  310 . The second layer  340  may be composed of a less rigid and compressible material such that a heat sink first contacting the second layer  340  may compress the second layer  340  before contacting the die  310 . Such a design may protect both the top surface of the die  310  and the die  310  edges by absorbing part of the force associated with the employment of a heat sink to the die  310 . The extender  350  may protect the die  310  edges and absorb the force of a heat sink as it is applied to the die, regardless of whether the die extender  350  is in contact with the die  310 .  
       FIG. 4  illustrates a cross-sectional view of an embodiment of a semiconductor package  400 . The package  400  includes a die  410  attached to a package substrate  420 . A die extender  450  includes a first component  440  disposed on the substrate  420  and a second component  430  that may be embedded in the first component  440 . The first  440  and second  430  components are typically composed of different materials as provided herein. The second component  430  may or may not be disposed directly on the substrate  420 . In an embodiment, both the first and second components  440 ,  430  are in contact with the substrate  420  and the first component  440  occupies a greater percentage of the surface area of the substrate  420  than the second component  430 . In another embodiment, both the first  440  and second  430  components are in contact with the substrate  420  and the second component  430  occupies a greater percentage of the surface of the substrate  420  than the first component  440 . In another embodiment, both the first  440  and second  430  components are in contact with the substrate  420  and occupy substantially equal percentages of the surface of the substrate  420 .  
      A die extender  450  comprising more than one component may be designed such that it protects the edges of the die  410 , and such that a heat sink applied to the package  400  first contacts one component made of a more compressible material, and then contacts the second component made of a less compressible material, or vice versa. In an embodiment, the second component  430  comprises a thickness such that a heat sink applied to the package  400  would contact the second component  430  prior to contacting the first component  440  and/or the die  410 . In another embodiment, the first component  440  comprises a thickness such that a heat sink applied to the package  400  would contact the first component  440  prior to contacting the second component  430  and the die  410 . In another embodiment, the first component  440  and second component  430  are designed such that the surface of the second component  430  is substantially planar with the surface of the first component  440 . In another embodiment, the first component  440 , the second component  430 , or both may be substantially planar with the surface of the die  410 .  
      In one example of a die extender illustrated by the embodiment of  FIG. 4 , a first component  440  may be more rigid than the second component  430 . A heat sink applied to the package  400  would first contact the second component  430 . The material and design of the second component  430  may be such that the pressure associated with applying the heat sink compresses the second component  430 . In this way the second component  430  acts to provide a cushion as the heat sink is applied to the die. The surface of the first component  440 , may be either substantially planar with the corresponding surface of the die  410 , or between the surface of the die  410  and the surface of the second component  430 . The level of the surface of the first component  440  may be set such that it absorbs some of the pressure stresses associated with applying the heat sink to the die  410 , while also permitting sufficient contact between the heat sink and die  410  for effective heat sinking to occur. In addition, the first component  440  may contribute to leveling the heat sink so that it makes flush contact with the die  410 . Thus, the first component  440  and second component may together provide a cushion and stabilizing effect for the package  400  and die  410  as a heat sink is applied.  
       FIG. 5  illustrates a cross-sectional view of an embodiment of a semiconductor package  500  as provided herein. The package  500  includes a die  510  and a die extender  530 . In an embodiment, the die extender  530  is at least as thick as the die  510 . As illustrated by the embodiment of  FIG. 5 , the die extender  530  may be thicker than the die  510 . In an embodiment, the die extender material may include a foaming agent so that a heat sink may compress the die extender  530  in order to make sufficient contact with the die. The foaming agent may be a separate layer or integral to the die extender material. The foaming agent may enhance the ability of the heat sink to compress the die extender  530  in order to make sufficient contact with the die  510 . The die extender  530  may be as thick as or thicker than the die  510  as long as effective contact may be made between the die  510  and a heat sink.  
      In an embodiment, a heat sink compound is applied between the die  510  and a heat sink. Appropriate heat sink compounds are known in the art. Examples include those supplied by Herbach and Rademan of Moorestown, N.J., and those supplied by All Electronics Corporation of Van Nuys, Calif. An example is a zinc oxide filled silicone that meets military specification (Mil-Spec) C-4713. In an embodiment, a metal layer is placed between the die  510  and a heat sink. A heat sink compound, metal layer, or a combination of a heat sink compound and metal layer may improve thermal contact between a heat sink and the die  510  and also help prevent the heat sink from damaging the die. Particles or contaminants trapped between a heat sink and the die  510  may be compacted against the die  510  by the heat sink causing damage to and cracks in the die  510 . A heat sink compound, metal layer, or a combination of a heat sink compound and metal layer is typically compressible and acts as a cushion to absorb such particles and contaminants and protect the die  510  from damage while also providing effective thermal contact between the die  510  and heat sink. In an embodiment, the metal layer is composed of a soft metal. In another embodiment, the metal layer may be comprised of aluminum, indium, copper, silver, gold, tungsten, titanium, iron, chrome, nickel or combinations thereof. In another embodiment, the metal layer may be a discontinuous layer or mesh. A soft heat sink compound or metal layer such as aluminum is designed to be supple enough to compensate for roughness, particles, and/or irregularities between the surface of a heat sink and the surface of the die  510  as the heat sink is pressed against the package  500 . Such softness promotes flush physical and effective thermal contact between the die  510  and heat sink.  
      In an embodiment, a method is provided including attaching an IC die to a package substrate and disposing a die extender around the perimeter of the die on the substrate, where the die extender frames the die and protects the die during handling. In another embodiment, a metal layer or heat sink compound is employed between a heat sink and an IC die. The metal layer or heat sink compound may be applied to a semiconductor package by any means known in the art. In embodiments, the metal layer is applied via sintering, sputtering, laser processing, or placing of a pre-formed metal piece. The metal layer may be removable after contact with a heat sink. The die extender is designed to provide protection to the IC die during various forms of handling of an IC package. Such handling may be any step known in the art of semiconductor manufacturing to result in mechanically contacting the die. Examples of such handling include clamping, testing, temperature cycling, electrically connecting, applying a heat sink compound, and applying a heat sink. Applying a heat sink may include direct contact with the die, or a heat sink compound or metal layer may be placed between the heat sink and die.  
      In another embodiment, a method of modifying a semiconductor package includes disposing a die extender on a package substrate such that the die extender frames and protects an IC die on the substrate. In embodiments, the die extender is at least as thick as the die and disposed on the substrate around the perimeter of the die. A die extender may be applied to a semiconductor package via any means known in the art. In an embodiment, the die extender may be applied by pouring into a cast, silk screening or screen printing, spraying, injection molding, painting, dispensing from a syringe, placing of a pre-formed piece, or combinations thereof. The die extender may be formed of various materials. Any material that may be formed to act as a die extender to prevent damage to an IC die may be appropriate. The rigidity of the die extender material may vary according to the type of material used and its compressibility. In embodiments, the die extender material comprises an epoxy component, a foaming agent, a thermal plastic, or combinations thereof. Appropriate foaming agents are known in the art, such as, for example, those supplied by Clariant Corporation of Holden, Mass. In another embodiment, the one or more die extender components are selected so that they possess coefficients of thermal expansion (CTE) such that no de-lamination occurs.  
      While embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are exemplary only, and are not intended to be limiting. Equivalent techniques and ingredients may be substituted for those shown, and other changes can be made within the scope of the present invention as defined by the appended claims. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims.