Abstract:
The invention encompasses microelectronic package lids, heat spreaders, and semiconductor packages comprising microelectronic lids or heat spreaders. In particular aspects of the present invention, a microelectronic lid comprises a material having a rectangular peripheral shape that defines 4 peripheral sides. Further, the lid has projecting peripheral rails along less than all of the peripheral edge. For instance, the lid can have projecting peripheral rails along only 2 of the sides. Alternatively, such microelectronic lid can be described as comprising a generally rectangular shape defining four peripheral edges, with two of the edges having a greater thickness than the other two edges.

Description:
TECHNICAL FIELD  
         [0001]    This invention pertains to microelectronic lid designs, heat spreader designs, and semiconductor packaging.  
         BACKGROUND OF THE INVENTION  
         [0002]    Modem semiconductor device packaging typically involves provision of a microelectronic lid over a semiconductor die (also referred to as a chip) to protect the die during transport. The microelectronic lid can be thermally conducted with the die so that heat generated from the die is dispersed into the lid. Accordingly, the lid can function as a heat spreader in addition to functioning as a protective cover for the die.  
           [0003]    A prior art semiconductor package is described with reference to FIGS. 1-4. Referring initially to FIG. 1, the package comprises a base  10  and a lid  30 , which are initially provided as separate pieces. Base  10  can comprise a substrate  12 , which can be a circuit-retaining construction, such as, for example, a circuit board. A semiconductor chip  14  is provided in electrical connection with the circuit of circuit-retaining construction  12 , and can, for example, be connected to such circuit through solder bead electrical interconnects (not visible in the view of FIG. 1). A sealant material  16  is provided around an outer periphery of circuit-retaining construction  12 , and can comprise, for example, an epoxy. The surface of base  10  that is shown in FIG. 1 will ultimately be an inner surface in a package construction formed with lid  30 .  
           [0004]    Referring next to lid  30 , such comprises a recessed surface  32  surrounded by a non-recessed peripheral portion  34 . Lid  30  also comprises a surface  36  that is in opposing relationship to surface  32 , and accordingly that is a hidden underside of lid  30  in the view of FIG. 1. The surface  32  of lid  30  will ultimately be an inner surface of the lid in a package formed with lid  30  and base  10 , and the surface  36  will be an outer surface of such package.  
           [0005]    [0005]FIG. 2 shows a top view of a package  40  comprising lid  30  and base  10 . A process step in formation of package  40  is to invert lid  30  from the configuration shown in FIG. 1, and to press the lid over base  10 . Lid  30  is sealed to base  10  by sealing peripheral portion  34  of lid  30  to the base with sealant material  16 .  
           [0006]    [0006]FIG. 3 shows a cross-sectional view through the package  40  of FIG. 2, and illustrates lid  30  joined with base  10 . Also visible in FIG. 3 are electrical interconnects  42  extending downwardly from chip  14  to electrically connect the chip with circuitry (not shown) retained in substrate  12 . Additionally, FIG. 3 shows a thermally conductive interface material  44  provided on chip  14  and thermally connecting lid  30  with chip  14  to allow heat dispersion from chip  14  into lid  30 . If material  44  were not present, or were replaced with a non-thermally conductive material, lid  30  would simply be a microelectronic lid. However, if material  44  is a thermally conductive material, lid  30  functions as a heat spreader, with the term heat spreader understood to indicate a construction that primarily spreads heat in two dimensions, rather than in three dimensions.  
           [0007]    [0007]FIG. 4 illustrates the package  40  of FIG. 3 attached to a heat sink  50  through a thermally conductive interface material  52 . Material  52  can comprise, for example, GELVET™, which is commercially available from Honeywell International, Inc. Heat sink  50  can comprise, for example, aluminum having a shape which incorporates numerous projecting fins and/or posts. The heat sink  50  is distinguished from a heat spreader, in that heat sink  50  disperses heat in three dimensions, rather than two.  
           [0008]    It can be problematic and costly to fabricate a lid having the complexity of lid  30 . Accordingly, it would be desired to develop improved microelectronic lid designs.  
         SUMMARY OF THE INVENTION  
         [0009]    The invention encompasses microelectronic package lids, heat spreaders, and semiconductor packages comprising microelectronic lids or heat spreaders. In particular aspects of the present invention, a microelectronic lid comprises a material having a rectangular peripheral shape that defines 4 peripheral sides. Further, the lid has projecting peripheral rails along less than all of the peripheral edge. For instance, the lid can have projecting peripheral rails along only 2 of the sides. Alternatively, such microelectronic lid can be described as comprising a generally rectangular shape defining  4  peripheral edges, with 2 of the edges having a greater thickness than the other 2 edges.  
           [0010]    The invention also encompasses heat spreaders having the above-described shapes of the microelectronic lids, and comprising materials with a thermal conductivity of at least 100 watts/meter-kelvin, preferably at least 150 watts/meter-kelvin and more preferably greater than 200 watts/meter-kelvin. In particular embodiments, the heat spreaders can comprise, consist or, or consist essentially of copper, and can have a thermal conductivity of about 350 watts/meter-Kelvin. In other embodiments, the heat spreaders can comprise, consist of, or consist essentially of aluminum and can have a thermal conductivity of about 220 watts/meter-kelvin. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Preferred embodiments of the invention are described below with reference to the following accompanying drawings.  
         [0012]    [0012]FIG. 1 is a diagrammatic view of a microelectronic package at a preliminary step of a prior art method for forming a package, and is shown comprising a lid which is separate from a base. The lid is shown in a bottom view, and the base is shown in top view.  
         [0013]    [0013]FIG. 2 is a view of a package comprising the lid and base of FIG. 1, and is shown in top view.  
         [0014]    [0014]FIG. 3 is a view of the FIG. 2 package shown along the line  3 - 3 .  
         [0015]    [0015]FIG. 4 is a view of the FIG. 2 package shown along the cross sectional view of FIG. 3, and shown at a processing step subsequent to that of FIG. 3.  
         [0016]    [0016]FIG. 5 is a diagrammatic bottom view of a microelectronic lid, or alternatively a heat spreader, encompassed by the present invention.  
         [0017]    [0017]FIG. 6 is a side view of the FIG. 5 lid.  
         [0018]    [0018]FIG. 7 is a view of the FIG. 5 lid in combination with a base, and shown at a preliminary step of forming a microelectronic package encompassed by the present invention. The base of FIG. 7 is shown in top view, while the lid is shown in bottom view.  
         [0019]    [0019]FIG. 8 is a top view of a package assembled utilizing the lid and base of FIG. 7.  
         [0020]    [0020]FIG. 9 is a cross-sectional view of the package of FIG. 8 shown along the line  9 - 9 .  
         [0021]    [0021]FIG. 10 is a cross-sectional view of the FIG. 8 package shown along the line  9 - 9 , and shown at a processing step subsequent to that of FIG. 9.  
         [0022]    [0022]FIG. 11 is a sideview of the FIG. 8 package.  
         [0023]    [0023]FIG. 12 is a sideview of the FIG. 8 package, and shown in accordance with an embodiment of the present invention different than that of FIG. 11.  
         [0024]    [0024]FIG. 13 is an isometric view of a piece of lid stock at a preliminary step of forming lids in accordance with a method of the present invention.  
         [0025]    [0025]FIG. 14 is an isometric view of the lid stock of FIG. 13 shown at a processing step subsequent to that of FIG. 13. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    A microelectronic lid or alternatively a heat sink encompassed by the present invention is described with reference to FIG. 5, and is shown generally as a lid  100 . Lid  100  comprises a generally rectangular shape (although other shapes are encompassed by the present invention, with such other shapes including, for example, circular, triangular, pentagonal, or other polygonal shapes). Lid  100  comprises a periphery defined by the four edges  102 ,  104 ,  106  and  108 . Lid  100  also comprises a recessed surface  110 , which is coextensive with the surface of edges  102  and  106 ; and raised rails  112  and  114  which extend along edges  108  and  104 . Additionally, lid  100  comprises a surface  120  (not visible in the view of FIG. 5) which is in opposing relation to surface  110 .  
         [0027]    A difference between the lid  100  of FIG. 5 and the prior art lid  30  (shown in FIG. 1) is in lid  100  having raised portions ( 112  and  114 ) extending along only a part of the periphery of the lid. In contrast, the prior art lid  30  has a raised portion ( 34 ) extending along its entire periphery.  
         [0028]    In the shown embodiment, lid  100  comprises a rectangular shape, and the raised peripheral portions are along two opposing sides ( 104  and  108 ) of the peripheral shape, while the remaining two sides ( 102  and  106 ) do not have raised portions extending along the predominate extent of such edges. In fact, the only raised portions associated with edges  102  and  106  are the terminal ends of raised portions  112  and  114 , with such ends being the regions of portions  112  and  114  that contact edges  102  and  106 . Such terminal portions of rails  112  and  114  are identified in FIG. 5 by the label  115 . Accordingly, edge  102  has an expanse  126  extending along the edge between terminal ends  115  of rails  112  and  114 , and such expanse  126  is not raised relative to surface  110 . Similarly, edge  106  has an expanse  128  extending between terminal ends  115  which is not raised relative to surface  110 .  
         [0029]    [0029]FIG. 6 shows a side view of lid  100  along the side  106 . Such side view illustrates the relationship of rails  112  and  114  relative to surface  110 , and further shows expanse  128  extending between rails  112  and  114 . Rails  112  and  114  define a groove  119  extending therebetween.  
         [0030]    Exemplary dimensions of the lid  100  of FIGS. 5 and 6 are a width “W” of about 35 ±0.35 millimeters; a length “L” of about 35 ±0.35 millimeters, and a thickness “T” of about 4.6 ±0.05 millimeters. Further, groove  119  can have a depth “D” of about 0.6 ±0.025 millimeters.  
         [0031]    Referring next to FIG. 7, lid  100  is shown adjacent a base  150 , Which is ultimately to be capped by lid  100  to form a package. Base  150  comprises four peripheral edges ( 151 ,  153 ,  155  and  157 ), and is similar to the base  10  of FIG. 1 in that it comprises a die  14  over a substrate  12 . Further, base  150  comprises a sealant  16  provided along peripheral edges of the substrate. However, a difference between base  150  of FIG. 7 and base  10  is that the sealant  16  is provided along only two of the peripheral edges of substrate  12  of base  150 , rather than along the four peripheral edges as was done with base  10 . Sealant  16  is provided along the two peripheral edges of the substrate  12  of base  150  that will ultimately contact raised edges associated with lid  100 .  
         [0032]    In a processing step subsequent to that of FIG. 7, lid  1   00  is placed over base  150 , and rails  112  and  114  are sealed against the base with sealant  16  to form a package. Such package is shown in FIG. 8 as a package  200 , aid specifically is shown in top view, with surface  120  of lid  100  being visible.  
         [0033]    Referring next to FIG. 9, package  200  is shown in cross-sectional view along the line  9 - 9  of FIG. 8. Such cross-sectional view shows solder beads  42  connecting die  14  with substrate  12 . Also, the cross-sectional view shows a layer  202  formed between die  14  and lid  100 . Laver  202  can comprise, for example, a thermally conductive material. If layer  202  comprises a thermally conductive material, then lid  100  can function as a heat spreader to dissipate heat generated by die  14 . In alternative embodiments, layer  202  can be omitted, or can be replaced with a non-thermally conductive material. In either of such alternative embodiments, lid  100  will function as a microelectronic lid to protect die  14 , but will generally not effectively dissipate heat from die  14 , and accordingly will not be utilized as a heat spreader.  
         [0034]    If lid  100  is utilized as a heat spreader, it preferably comprises a material with a thermal conductivity of at least 100 watts/meter-kelvin, and more perfectly at least 150 watts/meter-kelvin. In particular embodiments, lid  100  can comprise a material having a thermal conductivity in excess of 200 watts/meter-kelvin, such as, for example, copper or aluminum. In embodiments in which lid  100  comprises a metallic material, the lid can be nickel-plated. For instance, if lid  100  comprises copper or aluminum, it can be provided with a nickel-plating having a thickness of at least about  3  microns. The nickel plating can protect the underlying lid material from corrosion, and further can provide a reproducible surface for adherence to one or more thermal interface materials, as well as for adherence to epoxy.  
         [0035]    Referring next to FIG. 10, package  200  is illustrated after formation of a heat sink  50  over the package, and a thermal interface  52  connecting heat sink  50  with package  200 . Heat sink  50  and thermal interface  52  can comprise, for example, the materials described above with reference to the prior art construction of FIG. 4.  
         [0036]    Referring next to FIG. 11, the package  200  of FIG. 8 is shown in a side view. The chip ( 14 ) is not shown in the side view of FIG. 11 to simplify the drawing, although it is to be understood that chip  14  would be in the center of package  200  as illustrated by, for example, FIG. 9. The view of FIG. 11 shows that there is a gap  250  at the end of package  200  corresponding to a space between rails  112  and  114 . Such gap will typically be narrow, and in particular embodiments of the present invention can be left unfilled. However, if it is desired to fill gap  250  to prevent dirt or other contaminants from penetrating between lid  100  and substrate  150 , such can be accomplished by providing a filler material within the gap. Such is illustrated in FIG. 12, wherein gap  250  is filled with a filler material  260 . Filler material  260  can comprise, for example, epoxy. Filler material  260  can be provided after formation of package  200  by applying the filler material into gap  250 . Alternatively, filler material  250  can be provided before formation of package  200  at, for example, the processing step of FIG. 7, by providing the filler material at edges  151  and  153  of substrate  150 .  
         [0037]    The lid  100  of the present invention can be advantageous relative to prior art lids (such as, for example, the lid  30  of FIG. 1) in that lid  100  can be simpler to manufacture than the prior art lids. Lid  100  can be formed by, for example, the processing of FIGS. 13 and 14. Referring initially to FIG. 13, a bar  300  of lid stock is provided. The bar comprises dimensions “A”, “X”, and “Y”. Dimension “X” corresponds to a width along edge  106  of a finished lid  100  (FIGS. 5 and 6), and dimension “Y” corresponds to a thickness of rails  112  and  114  of a finished lid  100 . The dimension “A” is preferably longer than several integral lengths of edge  108  of a finished lid  100 .  
         [0038]    Referring next to FIG. 14, bar  300  is machined to form a groove  302  extending along a surface of the bar. Groove  302  defines rails  112  and  114  extending along edges of the lid stock. The stock can subsequently be cut along dashed lines  304  and  306  to define separated lids  100 ,  400  and  500 . The lids separated lids can subsequently be subjected to electroplating if a metal plating is desired over the material of the lids.  
         [0039]    Although FIGS. 13 and 14 illustrate a process wherein a lid stock bar  300  is machined to form groove  302 , it is to be understood that the invention encompasses alternative processing wherein the grooved material of FIG. 14 is formed by extruding a lid material into the shown shape.