Abstract:
The overall thermal resistance of a semiconductor device package containing a semiconductor die such as a VLSI IC die is reduced so as to improve the thermal performance of the package without any modification of the basic package structure. An extension of inactive or substantially inactive semiconductor material is added to the die adjacent to the boundary of a heat dissipating active circuit area on the die thereby increasing the effective heat transfer area of the die and establishing a heat spreading flow path to conduct heat away from the active circuit area.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to semiconductor devices such as integrated circuits, and more particularly to the thermal management of such devices.  
           [0003]    2. Description of the Related Art  
           [0004]    It is well known that many semiconductor packages, whether containing integrated circuits or individual devices such as power transistors, dissipate sufficient heat to require thermal management utilizing heat sinks. In particular, there has been increased emphasis on thermal management in integrated circuit (IC) packaging design stemming from the rapid growth of the number of active circuit elements per chip or die without a corresponding increase in die surface area. The resulting closer spacing of the circuit elements coupled with their higher switching speeds have led to dramatic increases in heat densities. The objective of thermal management in the design of IC packaging is to maintain the operating temperature of the active circuit or junction side of the IC die low enough (for example, 110° C. or below) to prevent premature component failure.  
           [0005]    Traditional methods of reducing the maximum junction temperature include lowering the thermal resistance attachment between the die and the package cover/heat spreader, improving heat sink efficiency, increasing package cover thermal conductivity, reducing the thermal resistance between the package cover and heat sink, and/or improving the cooling air flow inside the electronic system incorporating the IC package. For a device with extreme cooling requirements, an active or passive refrigeration system thermally coupled to the cover of the package is sometimes used. These traditional thermal management techniques, however, often increase the size and weight of the electronic system, can be expensive to implement and may risk compromising device reliability.  
           [0006]    When surfaces having different thicknesses and conductivities are joined together as in an IC device package, the concept of thermal resistance is useful for analyzing the heat flow. Thus, the overall thermal resistance can be modeled as the sum of three thermal resistances arranged in series between the junction or active circuitry side of the IC die and the outside surface of the cover: the thermal resistance of the die itself, the thermal resistance of the die/cover attachment interface material, typically a conductive epoxy, and the thermal resistance of the cover. The thermal resistances are functions of the thermal conductivities of the materials involved which, for the materials used in IC device packaging, have a broad range. Metals, of course, are the best conductors of heat while the conductive epoxies used, for example, to bond the die to the inside surface of the device cover are the poorest heat conductors. Thus, the thermal bottleneck in removing heat from an IC device is the interface between the IC die and the package cover. The problem of heat dissipation is exacerbated in the case of large, high power IC devices such as very large scale integration (VLSI) integrated circuit central processing units (CPUs). Because the dies of such units are relatively thin (for example, 0.76 mm thick), the IC die itself can provide only limited heat spreading. This can cause significant variations in local heat dissipation on the die surface leading to large surface temperature gradients in the case of a VLSI integrated circuit die having high power circuits along an edge or adjacent a corner thereof. This asymmetry, illustrated by prior art FIGS.  1 - 4 , tends to diminish or may completely eliminate heat spreading through the die in one or two directions.  
           [0007]    [0007]FIG. 1 shows a simplified cross section view of a conventional integrated circuit device package  10  containing an IC die  12  in the form of a flip-chip VLSI CPU. FIG. 2 is a bottom plan view of the IC die  12 . The IC device package  10  includes a package substrate  14  made of a ceramic such as alumina and having an upper surface  16  carrying the IC die  12 .  
           [0008]    The die  12  basically comprises a substrate  18  of semiconductor material, typically lightly-doped silicon, having a periphery  20  and opposed, parallel, major surfaces, namely, an upper surface  22  and a lower surface  24 , also referred to as the junction side or underside of the die  12 . In accordance with a typical example of a conventional VLSI CPU, the IC die  12  has a square configuration measuring 22×22 mm, with the periphery  20  thus comprising four edges  20   a - 20   d.    
           [0009]    With reference to FIG. 2, formed within an area  26  on the underside  24  of the semiconductor substrate  18  using well-known integrated circuit fabrication techniques, are numerous circuit features including regions defining junctions of active circuit elements. The circuit area  26  occupies essentially the entire surface area of the underside  24  of the semiconductor substrate  18 . Thus, the circuit area  26  has a boundary  28  substantially congruent with the periphery  20  of the semiconductor substrate  18 . (A very small inactive semiconductor border, that is, a border devoid of active circuit elements typically having a width of about 50-200μ for a 22×22 mm die, may exist around the area  26 .) In the example under consideration, high power active circuit elements, shown schematically as a block  29 , within the area  26  adjoin the portion of the boundary  28  immediately adjacent the edge  20   a.    
           [0010]    With reference again to FIG. 1, the IC die  12  is mechanically and electrically connected to the upper surface  16  of the package substrate  14  by means of an underfilled control collapse chip connection (C4)  30  coupling the junction side  24  of the die  12  to metalization on the upper surface  16  of the package substrate  14 . The space between the die underside  24  and the upper surface  16  of the package substrate is filled with a compliant, non-conductive epoxy  32 . As is known in the art, in C4 technology the die is flipped upside down to provide direct, very low inductance electrical connections between the circuit elements on the underside  24  of the die and the package substrate  14 .  
           [0011]    In the example under consideration, the underside  24  of the IC die has a land grid array (LGA) of signal pads or contacts in registration with a matching array of pads or contacts on the upper surface  16  of the package substrate  14 . The package substrate  14  typically comprises a multilayer assembly that interconnects the LGA on the upper surface of the package substrate  14  with a larger LGA of signal pads on the underside of the package substrate. This physically larger LGA may be connected to a host or higher assembly such as a printed circuit board  36  by means of an LGA interposer socket  38 .  
           [0012]    A cover  40  is attached to the package substrate  14  and includes an inner surface  42  defining with the upper surface  16  of the package substrate  14  an interior cavity or space  44  enclosing the IC die  12 . The cover  40  is fabricated of a heat conductive material such as aluminum silicon carbide. A compliant heat transfer interface  46 , such as a silver filled epoxy, is interposed between and thermally couples the upper surface  18  of the semiconductor die  12  and the inner surface  42  of the cover  40 .  
           [0013]    [0013]FIG. 3 shows an example of the asymmetric power distribution seen on the junction side  24  of the large integrated circuit die  12  depicted in FIGS. 1 and 2 and having high power circuits along the edge  20   a . As can be seen from the power map in FIG. 3, there are large variations in power density or heat flux across the integrated circuit die underside, with the highest concentration thereof existing along the edge  20   a  adjacent the high power circuits.  
           [0014]    Through the use of thermal modeling, the temperature distribution across the junction side of the IC die can be mapped, as shown (in ° C.) in FIG. 4, based on the power map of FIG. 3. As expected, the higher power densities along the edge  20   a  of the die  12  result in higher temperatures along that edge, and operating temperature gradients of 30 to 40° C., or more, across the die are not uncommon.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention reduces the overall thermal resistance of a semiconductor device package so as to improve the thermal performance of the package without any modification of the basic package structure.  
           [0016]    Broadly, in accordance with one exemplary embodiment of the invention, there is provided a semiconductor die comprising a pair of opposed parallel major surfaces and a periphery; an active circuit area within a boundary on one of the major surfaces of the semiconductor die, the active circuit area comprising at least one active circuit element that dissipates heat during operation; and a heat spreading extension disposed between at least a portion of the boundary and at least a portion of the die periphery adjacent the boundary portion, the extension being operable to establish a heat flow path to conduct heat away from the at least one heat dissipating active circuit element.  
           [0017]    Pursuant to another specific embodiment of the invention, there is provided a semiconductor package comprising a package substrate having an upper surface; a thermally conductive cover secured to the package substrate, the cover including an inner surface, the inner surface of the cover and the upper surface of the package substrate defining a space; and a semiconductor die enclosed within the space, the semiconductor die having a major surface and a periphery, the surface of the semiconductor die including an active circuit area comprising at least one active circuit element dissipating heat during operation of the semiconductor package, the active circuit area having a boundary, the surface of the semiconductor die being thermally coupled to the inner surface of the cover and wherein the die includes a heat spreading extension integral with the die, the heat spreading extension being disposed between the boundary of the active circuit area and the periphery of the die, the heat spreading extension being operable to establish a heat flow path to conduct heat away from the at least one active circuit element. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    Further objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments, below, when taken in conjunction with the accompanying drawings in which:  
         [0019]    [0019]FIG. 1 is a schematic side elevation view, in cross section, showing a conventional integrated circuit package;  
         [0020]    [0020]FIG. 2 is a bottom plan view of the junction side of the IC die incorporated in the package of FIG. 1, as seen along the line  2 - 2  in FIG. 1;  
         [0021]    [0021]FIG. 3 is a power density map for a conventional IC die such as that shown in FIGS. 1 and 2;  
         [0022]    [0022]FIG. 4 is the predicted junction temperature map for the power density map of FIG. 3;  
         [0023]    [0023]FIG. 5 is a schematic side elevation view, in cross section, of an integrated circuit package in accordance with a first preferred embodiment of the invention;  
         [0024]    [0024]FIG. 6 is an enlarged, side elevation view of a portion of the package shown in FIG. 5;  
         [0025]    [0025]FIG. 7 is a bottom plan view of the junction side of the IC die incorporated in the package of FIG. 1, as seen along the line  7 - 7  in FIG. 5;  
         [0026]    [0026]FIG. 8 is a graph showing a specific example of the cooling effect derived from the present invention;  
         [0027]    [0027]FIG. 9 is a bottom plan view of the junction side of an IC die in accordance with a second preferred embodiment of the invention; and  
         [0028]    [0028]FIG. 10 is a bottom plan view of the junction side of an IC die in accordance with a third preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Although the invention will be described in detail in the context of a flip-chip VLSI integrated circuit device package, it will be obvious to those skilled in the art that the invention has broader utility, being applicable to a wide range of semiconductor packages including those for individual high power density semiconductor devices such as high power transistors and laser diodes.  
         [0030]    In the description of FIGS.  5 , et seq., which follows, the same reference numerals used in connection with prior art FIGS. 1 and 2 will be used to designate like elements.  
         [0031]    [0031]FIG. 5 shows, in simplified form, a semiconductor device package  50  in accordance with a first, preferred embodiment of the invention, enclosing a semiconductor die  52  which, as before, may take the form of an integrated circuit such as a flip-chip VLSI CPU. Except for the die  52 , the package  50 , structurally and dimensionally, is basically the same as the conventional package  10 , thus including a package substrate  14  having an upper surface  16 ; a cover  40 ; and an interior space  44 . The IC die  52  device is mounted on the upper surface  16  of the package substrate  14  within the space  44  in the manner described earlier.  
         [0032]    Referring now also to FIGS. 6 and 7, the IC die  52  basically comprises a semiconductor substrate  53  including upper and lower major sides  54  and  56 , respectively, and a periphery  58 . In accordance with the specific example under consideration, the IC die  52  has a rectangular or square configuration in plan view, including four edges  58   a - 58   d . The underside  56  of the IC die  52  comprises the junction side of the die, that is, the side incorporating the active circuitry of the integrated circuit. As in the conventional IC described in connection with FIGS. 1 and 2, the active circuitry is contained within an area  60  having a boundary  62 . The area  60 , in accordance with the specific example under consideration, has a square or rectangular configuration bounded by four lines  62   a - 62   d  paralleling respective ones of the peripheral edges  58   a - 58   d  of the die  52 . The boundary lines  62   b - 62   d  substantially coincide with the edges  58   b - 58   d , respectively.  
         [0033]    The active circuit area  60 , including its dimensions and circuit contents, is conventional and the same as already described. Thus, high power dissipation active circuit elements, represented by a block  63 , are disposed along one the boundary line  62   a  paralleling the peripheral edge  58   a  of the die  52 .  
         [0034]    In accordance with the invention, the portion of the die between the boundary line  62   a  and the edge  58   a  comprises a heat spreading extension  64  of inactive semiconductor substrate material. The extension  64  extends along the entire length of the boundary line  62   a  and has a width, w, which may measure 1 mm by way of example and not limitation, for an IC die 0.76 mm thick and having an active circuit area  60  measuring 22×22 mm. The extension  64  serves during operation of the integrated circuit package  50  to establish a thermal energy flow path to spread and conduct heat away from the active circuit area  60  and to transfer that heat to the cover  40  via a heat transfer interface  66  for dissipation to the ambient environment. A similar heat transfer interface  68  couples the underside of the die and the upper surface  16  of the package substrate  14 .  
         [0035]    As before, the heat transfer interface  66  comprises a compliant, conductive epoxy such as a silver filled epoxy, while the interface  68  comprises a compliant, non-conductive epoxy. The thermal interfaces  66  and  68  above and below the IC die extend across the entire upper and lower major surfaces  54  and  56  of the integrated circuit die  52  including the heat-spreading extension  64  formed thereon. As shown in the enlargement of FIG. 6, heat, represented by the arrows  70 , is thus transferred from the active circuit area  60  into the extension  64  and from there through the heat transfer interface  66  to the cover  40  of the device package.  
         [0036]    The extension  64  is created during the manufacture of the die in the wafer form. During the wafer sawing operation, the wafer is cut in such a manner that an inactive margin of semiconductor remains adjacent to the high power circuit boundary  62   a  to form the extension  64 .  
         [0037]    In some cases this approach might increase the cost of the integrated circuit by reducing the number of dies that can be placed on a single wafer. However, any increased cost is outweighed by the device cooling made available through the present invention which reduces or may even eliminate the need for any of the cooling enhancements mentioned earlier.  
         [0038]    As will be evident to those skilled in the art, the cooling benefits afforded by the present invention will vary with the design and power distribution of the semiconductor device. FIG. 8 is a graph showing the cooling effect imparted by an extension of inactive semiconductor material for a 0.76 mm thick VLSI CPU of a specific design and a specific power distribution across an active circuit area having a high temperature region adjoining one of the boundaries. It will be seen that for the specific example that is the subject of FIG. 8, the addition of an extension of even a modest width, w, of, for example, 1 mm to 1.5 mm by itself significantly decreases the maximum junction temperature.  
         [0039]    [0039]FIG. 8 shows that for a given die thickness, the cooling benefit obtained diminishes as the width of the extension increases. Also, it will be evident that the increased amount of heat spreading provided by the extension is a function of both the die thickness and the width of the extension. Thus, as another example, a die having a thickness of 0.38 mm would benefit substantially from an extension having a width of only 0.4 to 0.5 mm.  
         [0040]    [0040]FIGS. 9 and 10 show alternative, preferred embodiments of the invention. FIG. 9 depicts a die  80  having extensions  82  and  84  of inactive semiconductor material projecting from two adjacent boundaries  86  and  88  of an active circuit area  90 , as well as from the corner  92  shared by those boundaries. FIG. 10 shows a square die  100  having four extensions  102 - 105  provided around an entire active circuit area  106 , including the corners thereof. The addition of extensions about the entire boundary of an active circuit area may be especially advantageous for small, high power semiconductor devices such as high power transistors and laser diodes that are individually packaged. Additional extension configurations will suggest themselves to those skilled in the art depending upon the heat transfer requirements of a particular device.  
         [0041]    It will be evident to those skilled in the art that instead of forming the die extension(s) from inactive semiconductor material that is completely inactive, low power dissipation circuit elements could be carried by the extension. Significant heat spreading benefits could thereby still be obtained.