Abstract:
An arrangement for improving the cooling efficiency of semiconductor chips. One embodiment is to construct a vapor chamber with one compliant surface for improving the efficiency of transferring heat from a semiconductor chip to the vapor chamber, and another embodiment is to construct a vapor chamber with the chip substrate such that the chips are embedded inside the vapor chamber. One surface of the vapor chamber has a flexible structure to enable the surface of the vapor chamber to be compliant with the surface of a chip or a heat sink device.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. Ser. No. 11/513,786, filed Aug. 31, 2006, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to a method of semiconductor chip cooling. The invention further provides for an arrangement to improve the cooling capability of semiconductor chips, as well as the requisite mechanical structure to achieve an efficient degree of cooling. 
         [0003]    A problem which is prevalent in the electronics industry, resides in the difficulties encountered in transferring heat generated from a semiconductor chip to a heat sink arrangement or device for cooling of the semiconductor chip. The presently encountered increase in semiconductor chip power and power density are frequently accompanied by a rise in the thermal resistance between the semiconductor chip and a heat sink device. The prevalent traditional employment of a thermal grease may readily reach its maximum upper limit to provide a low thermal resistance, which is demanded because of its low thermal conductivity and the requirement for providing a close-fitted mechanical structure so as to ensure a satisfactory performance thereof. An improved semiconductor chip thermal interface, as well as a superior mechanical interface between the semiconductor chip and its heat sink device while subjecting the chip to only a low mechanical stress, is therefore a need which must be met in the technology. 
       DISCUSSION OF THE PRIOR ART 
       [0004]    Currently, various publications are directed to solving of the problems encountered in transferring heat from heat-generating components to heat sink structures through the employment of diverse heat conveying and spreading methods of which the following are representative. 
         [0005]    Saaski, et al., U.S. Pat. No. 4,833,567, disclose a method of placing electronic components inside an integral heat pipe which consists of a condenser cap mounted on a substrate in order to form a sealed pipe chamber. Multi-layered wicks are placed between the condenser and the electronic components, so as to facilitate the transfer of heat away from the electronic components. 
         [0006]    Shanker, et al., U.S. Pat. No. 4,912,548, are concerned with a method of inserting a heat pipe through the lid of a semiconductor chip housing whereby the heat pipe terminates within the housing cavity at the hot end of the heat pipe, and enables the transfer of heat to a cold end thereof. 
         [0007]    Saito, et al., U.S. Pat. No. 4,796,155, teach a method of housing a solid state device inside a coolant vessel containing a liquid coolant with the presence of a vapor space, and which is adapted to provide a coolant vapor to cool the solid state device. 
         [0008]    Wiech Jr., U.S. Pat. No. 4,519,447, discloses two methods for cooling a substrate, one method employing a flat vapor chamber for heat spreading, and the other method utilizing a magnetohydrodynamically-driven liquid cooling plate to form a heat transferring path for cooling the substrate. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, pursuant to the present invention, a first method and arrangement for cooling a semiconductor chip resides in locating a vapor chamber with a compliant surface facing the chip, which is located underneath the chamber. A second method positions a chip inside a semiconductor vapor chamber that possesses compliant chamber walls, whereby the first method can be applied to a chip package that is constituted as a separate entity, whereas the second method provides an improvement thermally, but requires modifications of the chip package to include the vapor chamber structure. 
         [0010]    In accordance with the foregoing, pursuant to the first method or embodiment of the vapor chamber with a compliant surface, the chamber is mounted on a semiconductor chip package, with a center member on one surface of the vapor chamber being encompassed by a corrugated or resiliently flexible structure enabling the center member to move up and down relative to the therebeneath located chip surface. The center member possesses a good thermal interface relative to the chip surface from which heat generated by the chip is transferred, thereby cooling the chip. 
         [0011]    Pursuant to the second method, the chip packaging substrate is employed to form the vapor chamber, with the vapor chamber walls and the semiconductor chip being mounted on the substrate within the vapor chamber. The vapor chamber has an encompassing corrugated structure enabling the upper part of the vapor chamber to be reciprocated up and down to match the surface of a heat sink device, and to provide a superior thermal contact in order to efficiently remove heat from the semiconductor chip. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Reference may now be made to the following detailed description of preferred embodiments of the invention, as illustrated in the accompanying drawings; in which: 
           [0013]      FIG. 1  illustrates a generally diagrammatic cross-sectional view of a compliant vapor chamber mounted on a semiconductor chip package; 
           [0014]      FIG. 2  illustrates a cross-sectional view of a compliant vapor chamber with a semiconductor chip embedded inside thereof; and 
           [0015]      FIG. 3  illustrates a cross-sectional view of two compliant vapor chambers sharing a common heat sink device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Referring in particular to the drawings,  FIG. 1  illustrates a cross-sectional view of an exemplary embodiment of a vapor chamber  10  with a compliant surface mounted on a chip package. As shown in the figure, the vapor chamber  10  includes a cavity  12  having a wick structure  14  constituted of suitable permeable materials, such as thin meshes, fibers, foams and other porous material compositions, which are adhered on the inner surface of the cavity walls of the vapor chamber  10 , the latter of which is partially filled with a volatile fluid  16 , for example, such as water, ethanol, butane, and so forth. The housing  18  forming the vapor chamber  10  is constituted of suitably selected thermally conductive materials that are compatible with the volatile fluids. One side  20  of the vapor chamber  10  is a flat surface whereas the opposite side thereof has a center member  22  at the middle, which is surrounded by a flexible corrugated ring  24 . The flexible structure of the corrugated ring  24  allows the center member  22  to move slightly up and down or tilted in compliance with the semiconductor chip surface  26 , which is located underneath the vapor chamber  10 . Because of this structure, the gap between the center member  22  and the chip  28  can be made as thin and uniformly as possible in order to ensure a good thermal interface with the chip  28 , to which center member  22  there is transferred heat generated by the chip  28 . A section of thermal interface material  30  such as, for example, low melting point alloys of indium-bismuth-tin or indium-bismuth, is inserted between the center member  22  and the chip  28  so as to further reduce the thermal resistance between the center member  22  and the chip  28 . When the chip package  40  is heated to a temperature which is higher than the melting point of these alloys; for instance, about 80° C., the alloys will melt and be compressed by the center member  22  based on the action of three forces; namely, from the spring force of the corrugated ring  24 ; springs  32 , which are located inside the vapor chamber  10  at the location of the center member  22  and bearing thereagainst in a direction towards chip  28 , and the internal vapor pressure which is generated in the vapor chamber  10 . An elastomer barrier  34  extending about the chip  28  under the vapor chamber  10  prevents the residues of the melting alloys  30  from moving or to flow away from the chip package  40 . Heat pipes  42  and  44 , which are soldered to the perimeter of the vapor chamber  10 , convey heat from the vapor chamber  10  to a heat sink device (not shown) which, if desired, may be located in an area distant from the chip package  40 . In this illustrated arrangement, heat generated in the chip  28  is transferred to the center member  22  of the vapor chamber  10  through the interface material  30 , which may be in the form of a layer. The volatile fluid  16  inside the vapor chamber  10  near the center member  22  absorbs the heat and vaporizes. The resultant vapor stream then travels across and through the cavity  12  and condenses in the wick structure  14  on the inner wall surfaces of the vapor chamber  10 . The fluid  16  which condenses in the wick structure then flows back towards the center member  22  through the capillary force of fluids acting on the wicks. Heat is therefore released and transferred to the heat pipes  42  and  44 , which, in turn, convey the heat to the distant (or proximate) heat sink device. For purposes of clarity, the ports for the evacuation from and filling of volatile fluids into the vapor chamber  10  are not shown in the drawing figure. 
         [0017]    The housing  18  further includes an outer wall portion  15 , as shown in  FIG. 1 . The outer wall portion  15  includes a recessed portion  15   a.  The recessed portion  15   a  is defined by a central portion of the outer wall portion  15 , which is positioned between two end portions  15   b.  The central portion, and thereby the recessed portion  15   a,  has a first thickness, and each of the two end portions have a second thickness greater than the first thickness, such that the two end portions  15   b  define the recessed portion  15   a  therebetween. 
         [0018]    The arrangement in  FIG. 2  shows a cross-sectional view of another embodiment of an integrated vapor chamber  50  on a chip package  52 . In this package  52 , there is employed a packaging substrate  54  and the vapor chamber walls  56  to form vapor chamber  50 . A semiconductor chip  58  is mounted on the substrate  54  within the vapor chamber  50 . The packaging substrate  54  is of a vacuum-tight structure, and has vias and electrically conductive wires or traces (not shown) for connecting the semiconductor chip  58  to connection pads  60 , which are located on the other side  62  of the substrate  54 . The vapor chamber walls  56  have a bellows structure  64  extending thereabout for a portion of the height thereof, whereby the top part  66  of the vapor chamber  50  can be moved slightly up and down or tilted in order to be able to match or conform with the surface of a heat sink device (not shown). The top part  66  of the vapor chamber  50  can be selectively made of thermally-conductive materials, such as copper, copper alloys, aluminum, and the like. This resulting compliant constructional mechanism provides for a good thermal contact between the top part  66  of the vapor chamber  50  and the heat sink device. Wicks  68  which are arranged inside the vapor chamber  50  are adhered to the inner wall surface  70  of the vapor chamber  50 , as well as to the surface of the backside  72  of the chip  58 . There are basically two ways in which to arrange the wicks  68  in order to accommodate this compliant feature, one of which is shown in the figure and comprises two wick pieces  68   a  and  68   b,  which are overlapped at their ends such that fluids flowing in wick pieces  68   a  can pass to wick pieces  68   b  at the overlapping regions  68   c.  Alternatively, the wicks  68  at the overlapping regions  68   c  can be made from flexible materials such as fibers or thin meshes. A port  74  located on one wall  56  is for evacuation of the vapor chamber  50  and for filling the chamber with a predetermined amount of electrically-nonconductive volatile fluids that are compatible with the chamber materials; for example, such as water, ethanol, butane, or the like. Heat generated in the chip  58  heats up the fluids in the wick  68   b  proximate the chip area and causes the fluids to vaporize. The resultant vapor then flows across the vapor chamber and condenses in the wicks  68  within the region of the top part  66  of the vapor chamber  50 . The heat released from the process of condensation is transferred to the heat sink device (not shown), which is preferably located in contact with the upper surface of the top part  66  of the vapor chamber  50 . The condensed fluids will then flow back down into the region near the chip  58  and the vaporization and condensation cycle continues in a repetitive mode. 
         [0019]    As disclosed in  FIG. 3 , this embodiment shows the cross-sectional view of two integrated vapor chamber chip packages  52 , as in  FIG. 2 , sharing a common heat sink device  80  located on top thereof. The two integrated vapor chamber-chip packages  52  each have, respectively, a similar structure as the one shown in  FIG. 2  and are designated by the same reference numerals. These two packages  52  are soldered to one common printed-wiring board  82 . The common heat sink device  80 , when placed on top of these two packages  52  can be constituted from a liquid-cooled cold plate or from an air-cooled heat sink. Because of the compliant feature of the integrated vapor chamber-chip package  52 , a good thermal contact between the top part  66  of each of the vapor chambers  50  and the heat sink device  80  is independently ensured for each separate chamber-chip package  52 . A piece of thermal interface material (now shown in the figure) can be placed between the common heat sink device  80  and the integrated vapor chamber chip packages  52 . 
         [0020]    While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but to fall within the spirit and scope of the appended claims.