Abstract:
Heat sinks having a mounting surface with a coefficient of thermal expansion matching that of silicon are disclosed. Heat pipes having layered composite or integral composite low coefficient of expansion heat sinks are disclosed that can be mounted directly to silicon semiconductor devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/074,987, filed Mar. 29, 2011, which published as Publication No. 2011/0176276, which is a continuation of U.S. patent application Ser. No. 11/065,465, filed Feb. 24, 2005, which published as Publication No. 2005/0139995, which is itself a continuation-in-part of U.S. patent application Ser. No. 10/924,586, filed Aug. 24, 2004, now U.S. Pat. No. 7,048,039, which is itself a continuation of U.S. patent application Ser. No. 10/458,168, filed Jun. 10, 2003, now U.S. Pat. No. 6,793,009, the content of each of which is incorporated herein by reference in its entirety. This application also incorporates herein by reference U.S. Provisional Patent Applications Nos. 60/561,436, filed Apr. 12, 2004, and 60/574,158, filed May 25, 2004, in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to heat sinks having mounting surfaces for semiconductors, and more particularly to such heat sinks which include one or more mounting surfaces having a coefficient of thermal expansion that matches or nearly matches silicon. 
       BACKGROUND 
       [0003]    It is known that certain classes of semiconductor devices consume substantial amounts of power, which results in excess thermal energy that then must be transferred to the ambient environment. This waste heat is typically communicated through a variety of thermal interfaces, heat spreaders, and structural elements prior to being rejected into the ambient atmosphere by a heat sink. Since heat is often dissipated to room temperature air, and the silicon constructed semiconductor has a finite upper bound on its operating temperature, package-related thermal resistance is becoming a limiting factor in the ability to dissipate the waste heat. 
         [0004]    The removal of package elements and interfaces will reduce package thermal resistance, and allow the semiconductor device to either run cooler or dissipate more power. However, many of these elements are required in order to provide a match between the relatively low coefficient of thermal expansion (CTE) of silicon and the relatively high CTE of the metal comprising the heat sink, rather than for best thermal performance. This match needs to be maintained in order to prevent build-up of stress, as well as subsequent damage due to failure of the relatively brittle silicon component. Thus, there are the competing structural requirements of providing a layer of material to provide a CTE match while at the same time needing to bring the heat transfer structure into intimate physical contact with the heat generating structure. 
         [0005]    Matching may be achieved by at least two methods: the use of an alloy substrate such as copper/tungsten whose CTE matches or nearly matches that of the silicon, or through the use of a ductile braze alloy between the silicon and the remaining package elements. Either method prevents transmission of stresses due to mismatched CTE through the interface to the silicon device. Some disadvantages of the alloy substrate include expense, unfavorable machining and stamping characteristics, and a fairly low thermal conductivity. Some disadvantages of the ductile braze alloy include a limited fatigue life, which eventually results in failure due to delamination of the joint. This tendency is exacerbated by the service conditions of most high power devices. Such devices almost always operate under conditions of periodic fluctuating electrical load, which leads to periodic fluctuations in thermal load and mechanical stresses in the joint. 
         [0006]    An alternative method involves the use of direct bond copper (DBC) aluminum nitride (AlN) in sheet form. This material is a “sandwich” comprised of a single layer of aluminum nitride and two outer layers of OFE copper foil. The copper layers are first oxidized, and then pressed against the AlN at high temperature in a neutral atmosphere. This process causes the oxide to diffuse into the AlN and bonds the copper sheets tightly to the AlN inner layer. Since the copper layers are relatively thin and are in an annealed state due to the high processing temperature, the CTE of the resulting assembly is largely governed by that the of the AlN. 
         [0007]    None of the foregoing techniques have been found to be completely satisfactory or have been successfully applied to heat pipe cooling devices. 
       SUMMARY 
       [0008]    In one embodiment, a heat transfer device generally includes an interior chamber defined at least in part by a layered-composite wall. The layered-composite wall includes a first layer of material comprising a coefficient of thermal expansion that is substantially similar to the coefficient of thermal expansion of silicon. The first layer is disposed between and directly engages second layers of material comprising a coefficient of thermal expansion greater than the coefficient of thermal expansion of silicon. The wall has a periphery that is out of plane with respect to a remainder of the wall. 
         [0009]    In another embodiment, a heat pipe generally includes a body defining an interior chamber, a wick disposed on portions of the body that define the interior chamber, and a working fluid. The interior chamber is defined at least in part by a layered-composite wall. The layered-composite wall includes a first layer of material comprising a coefficient of thermal expansion that is substantially similar to the coefficient of thermal expansion of silicon. The first layer is disposed between and directly engages second layers of material comprising a coefficient of thermal expansion greater than the coefficient of thermal expansion of silicon. The wall has a periphery that is out of plane with respect to a remainder of the wall. 
         [0010]    In a further embodiment, a heat pipe generally includes a body defining an interior chamber, a wick disposed on portions of the body that define the interior chamber, and a working fluid. The interior chamber is defined at least in part by a layered-composite wall of molybdenum disposed between layers of oxygen-free electronic copper foil. The wall has a periphery that is out of plane with respect to a remainder of the wall. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    These and other features and advantages of the present invention will be more fully disclosed in, or rendered obvious by, the following detailed description of the preferred embodiments of the invention, which are to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein: 
           [0012]      FIG. 1  is a partly exploded, elevational view of a CTE-matched heat pipe formed in accordance with one embodiment of the present invention; 
           [0013]      FIGS. 2-4  are cross-sectional perspective views of the CTE-matched heat pipe of  FIG. 1 ; 
           [0014]      FIG. 5  is a broken-away cross-sectional view of a portion of the low CTE base illustrated in  FIGS. 2-4 ; 
           [0015]      FIG. 6  is a perspective view of a composite base comprising a high CTE portion and a complementary low CTE insert portion positioned for placement within an opening; 
           [0016]      FIG. 7  is an exploded cross-sectional view of a composite base, as taken along lines  7 - 7  in  FIG. 6 ; 
           [0017]      FIG. 8  is a cross-sectional view of the assembled composite base shown in  FIG. 7 ; 
           [0018]      FIG. 9  is a cross-sectional view of another embodiment of composite base having a wick applied to a surface of a low CTE insert portion; 
           [0019]      FIG. 10  is a cross-sectional view of yet another embodiment of composite base having a wick overlying a low CTE insert portion; 
           [0020]      FIG. 11  is a cross-sectional view of a further embodiment of a CTE-matched base; 
           [0021]      FIG. 12  is a cross-sectional view of tower heat pipe having a low CTE composite insert positioned within a high CTE base; 
           [0022]      FIG. 13  is a perspective view of another embodiment of composite base having a plurality of low CTE inserts; 
           [0023]      FIG. 14  is a perspective view of a planar heat pipe heat spreader formed in accordance with another embodiment of the present invention; 
           [0024]      FIG. 15  is a cross-sectional view of the embodiment of composite base shown in  FIG. 14 , as taken along lines  15 - 15  in  FIG. 14 ; 
           [0025]      FIG. 16  is an enlarged view of the cross-section shown in  FIG. 15 ; 
           [0026]      FIG. 17  is a perspective view of a planar heat pipe comprising a composite wall formed in accordance with another embodiment of the invention; 
           [0027]      FIG. 18  is a cross-sectional view of the planar heat pipe shown in  FIG. 17 , as taken along lines  19 - 19  in  FIG. 17 ; 
           [0028]      FIG. 19  is an enlarged cross-sectional view of the interior wall structures of a planar heat pipe formed in accordance with the present invention; 
           [0029]      FIG. 20  is a perspective view of a semiconductor device mounted on the planar heat pipe shown in  FIGS. 17-18 ; 
           [0030]      FIG. 21  is a cross-sectional view of the planar heat pipe shown in  FIG. 20 , as taken along lines  21 - 21  in  FIG. 20 ; 
           [0031]      FIG. 22  is an enlarged cross-sectional view of the interior wall structures of a planar heat pipe formed in accordance with the present invention; 
           [0032]      FIG. 23  is a perspective view of a planar heat pipe comprising a composite wall and a low CTE insert formed in accordance with another embodiment of the invention; 
           [0033]      FIG. 24  is a cross-sectional view of the planar heat pipe shown in  FIG. 23 , as taken along lines  24 - 24  in  FIG. 23 ; 
           [0034]      FIG. 25  is an enlarged cross-sectional view of the interior wall structures of a planar heat pipe formed in accordance with the present invention; 
           [0035]      FIG. 26  is a perspective view of a planar heat pipe comprising a composite wall and a low CTE insert formed in accordance with another embodiment of the invention; 
           [0036]      FIG. 27  is a cross-sectional view of the planar heat pipe shown in  FIG. 26 , as taken along lines  27 - 27  in  FIG. 26 ; 
           [0037]      FIG. 28  is an enlarged cross-sectional view of the interior wall structures of a planar heat pipe formed in accordance with the present invention; 
           [0038]      FIG. 29  is a cross-sectional view of yet a further embodiment of a planar heat pipe having a composite wall structure formed in accordance with the present invention; and 
           [0039]      FIG. 30  is an enlarged cross-sectional view of the interior wall structure of a planar heat pipe formed in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    This description of preferred embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description of this invention. The drawing figures are not necessarily to scale and certain features of the invention may be shown exaggerated in scale or in somewhat schematic form in the interest of clarity and conciseness. In the description, relative terms such as “horizontal,” “vertical,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing figure under discussion. These relative terms are for convenience of description and normally are not intended to require a particular orientation. Terms including “inwardly” versus “outwardly,” “longitudinal” versus “lateral” and the like are to be interpreted relative to one another or relative to an axis of elongation, or an axis or center of rotation, as appropriate. 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. The term “operatively connected” is such an attachment, coupling or connection that allows the pertinent structures to operate as intended by virtue of that relationship. In the claims, means-plus-function clauses are intended to cover the structures described, suggested, or rendered obvious by the written description or drawings for performing the recited function, including not only structural equivalents but also equivalent structures. 
         [0041]    Referring to  FIGS. 1-4 , a CTE-matched heat pipe  5  formed in accordance with one embodiment of the present invention includes a body  8 , a wick  12 , a working fluid  13 , and a base  15 . More particularly, body  8  may comprise a cylindrical tube formed from a highly thermally conductive metal, e.g., copper or its alloys or nickel or its alloys such as Monel (an alloy of nickel and copper) which could be incorporated into the structure with no significant changes in design or fabrication method. A vapor space is defined by a central passageway  20  extending along the longitudinal axis of body  8 . Body  8  includes a bottom end  22  and a top end  24 . Top end  24  is pinched off or otherwise sealed at a fill tube  26  during manufacture. Wick  12  is preferably formed from a brazed copper powder that is distributed throughout the inner surface of body  8  that defines central passageway  20  at bottom end  22 . Although not preferred, wick  12  may be distributed throughout the inner surface of body  8  at top end  24 , and may also comprise adjacent layers of screening or a sintered powder structure with interstices between the particles of powder, having an average thickness of about 0.1 mm to 1.0 mm. 
         [0042]    In one preferred embodiment of the present invention, no wick structure is present at top end  24  (the condenser region of heat pipe  5 ). This is due in large part to the fact that gravity will drive the return of condensed working fluid  13  in the particular orientation shown in  FIGS. 1-4 . A wick structure may be incorporated in top end  24 , i.e., in the condenser region of heat pipe  5 , in order to provide return of condensate when the evaporator portion of the heat pipe is oriented so as to be above the condenser region. A wick structure in top end  24  may also reduce the temperature drop associated with condensation, as well as improve performance of the device, even when the wick is not required to return the working fluid. 
         [0043]    Wick  12  may also include a screen or grooves integral with the inner surface of body  8 . Also, a plastic-bonded wick in the evaporator and condenser regions of heat pipe  5  may be produced simultaneously and as a contiguous structure after body  8  is brazed to base  15 . This would provide a contiguous fluid conduit between the evaporator and condenser regions of heat pipe  5 , which is advantageous when the evaporator is elevated. This feature may be met with a screen wick by “pushing” the screen wick into an annular gap  28  located between bottom end  22  and base  15 . 
         [0044]    Working fluid  13  may comprise any of the well known two-phase vaporizable liquids, e.g., water, alcohol, Freon, methanol, acetone, fluorocarbons or other hydrocarbons, etc. CTE-matched heat pipe  5  is formed according to the invention by drawing a partial vacuum within body  8 , and then back-filling with a small quantity of working fluid  13 , e.g., just enough to saturate wick  12  just prior to final sealing of body  8  by pinching, brazing, welding or otherwise hermetically sealing fill tube  26 , once base  15  is mounted to bottom end  22  of body  8 . The atmosphere inside heat pipe  5  is set by an equilibrium of liquid and vapor. 
         [0045]    Base  15  comprises a plurality of layers of selected materials so as to form a layered-composite having a low CTE, i.e., a CTE that nearly matches the CTE of a semiconductor, such as about 6.5 or less for silicon ( FIG. 1 ). For example, base  15  may be formed from a direct bond copper (DBC) aluminum nitride. Base  15  may comprise a variety of shapes that could be dictated by both the geometry of the semiconductor device  30  that is to be cooled by CTE-matched heat pipe  5 , or the shape of bottom end  22  of body  8 . Base  15  is fastened directly to bottom end  22  of body  8  without the use of intermediate layers of CTE matching materials or ductile brazes. A base  15  formed from DBC aluminum nitride possesses several advantages that make it attractive for use as an interface to silicon semiconductor devices and substrates. As no interposing intermediate layers of CTE matching materials or ductile brazes are needed, bottom end  22  of CTE-matched heat pipe  5  will be arranged in intimate thermal communication with semiconductor device  30 . The interface between bottom end  22  and semiconductor device  30  will also be significantly more resistant to thermal cycling and thermal fatigue. DBC aluminum nitride base  15  comprises high thermal conductivity, both in-plane and through-thickness, and its conductivity approaches that of aluminum. Thus, the construction of the present invention allows bottom end  22  of CTE-matched heat pipe  5  to approach the chip more closely, i.e., more closely than any method other than direct die contact or direct liquid cooling, so that the package thermal resistance is as low as possible. 
         [0046]    In another embodiment, base  31  may include a plurality of layers to form a layered-composite  38  comprising a layer of molybdenum  37  having a top surface  39  and a bottom surface  40  ( FIG. 5 ). A first layer  42  of OFE copper foil is disposed over top surface  39  and a second layer  43  of OFE copper foil is disposed over bottom surface  40  so as to form layered-composite  38  ( FIGS. 2-5 ). In this way, a layered composite is formed comprising a first layer  42  of relatively high CTE material (i.e., a CTE higher than that for silicon), a second layer  43  of relatively high CTE material (i.e., a CTE higher than that for silicon), and an intermediate layer  37  of relatively low CTE material, thus forming layered-composite  38  having an internal structure comprising high CTE material/low CTE material/high CTE material. The CTE of such a layered-composite is often in a range from about 2.5 to about 10, with a range from about 3 to about 6.5 being preferred for most silicon applications. 
         [0047]    When the present invention comprises a layered-composite  38  formed from layers of copper/molybdenum/copper, a thickness ratio of 13%/74%/13% has been found to provide adequate results. A copper/molybdenum/copper layered-composite  38  comprises mechanical properties that are suitable for higher temperature processing. This allows a silicon die to be attached to base  31 , via soldering, without structural instability which may cause the silicon to crack or break. 
         [0048]    Table 1 below presents thermal conductivity and CTE properties of different common materials that may be arranged as a layered-composite  38  in conformance with the present invention. In tower applications, it is preferred that the high CTE layers of material be selected so that base  15  may be fastened directly to bottom end  22  of body  8  without the use of any intermediate low CTE materials. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Coefficient Thermal Expansion 
               
               
                   
                 Material 
                 (ppm/° C.) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Silicon Carbide 
                 2.6 
               
               
                   
                 Silicon 
                 2.6 
               
               
                   
                 Molybdenum 
                 4.9 
               
               
                   
                 Graphite 
                 5 
               
               
                   
                 Beryllium Oxide 
                 8 
               
               
                   
                 Annealed Copper 
                 16.4 
               
               
                   
                 Aluminum Nitride 
                 3.6 
               
               
                   
                 80Mo20Cu 
                 7.2 
               
               
                   
                 75W25Cu 
                 10.2 
               
               
                   
                 33Cu/74Mo/33Cu 
                 10 
               
               
                   
                 13Cu/74Cu/13Cu 
                 6.5 
               
               
                   
                   
               
             
          
         
       
     
         [0049]    A brazed wick  33  may be formed on the inner surface of base  15  or  31 . Depending upon the heat load and particular power density, other wick structures may be appropriate. Examples of such structures include screen bonded to the heat input surface by spot-welding or brazing, a monolayer of powder metal, grooves cut in the copper layer of base  31 , or an array of posts. Furthermore, it is also anticipated that a plastic-bonded wick may be substituted for the brazed copper wick. 
         [0050]    In practice, semiconductor  30  is mounted to the bottom surface of base  31 . Heat from semiconductor  30  is conducted through base  31  into bottom end  22  of heat pipe  5 . The heat causes working fluid  13  in wick  12  to evaporate. The vapor travels through central passageway  20  to condenser region  35  of body  8 . At condenser region  35 , the vapor contacts the inner surface of body  8 , condenses, and gives up its latent heat through condensation. Working fluid  13  then returns to bottom end  22  by either gravity, or through the capillary action in a portion of wick  12  on the inner surface of body  8  at condenser  35 . 
         [0051]    As shown in  FIGS. 1-4 , fins  36  or other suitable extended surfaces may be mounted to body  8  at condenser region  35  to convey the heat to the ambient environment. It is anticipated that other fin types and structures are possible, including a folded fin wrapped around a cylindrical heat pipe envelope, an array of plate fins mounted radially around the condenser, or an array of fins mounted to the top of the device. 
         [0052]    Referring to  FIGS. 6-12 , a base  44  is also provided by the present invention in which a relatively low CTE layered-composite insert  45  is positioned within a relatively high CTE cold plate  50 , such as a copper plate. An opening  55  is formed within cold plate  50  that includes a counter-sunk region that provides an annular ledge  60  and a substantially vertical wall  62  ( FIGS. 6-8 ). Layered-composite insert  45  is positioned within opening  55  and fixedly fastened in intimate thermal communication with annular ledge  60  and vertical wall  62  so as to complete base  44 . Layered-composite insert  45  and cold plate  50  may be bonded together using conventional methods, such as brazing, soldering, adhesives, or direct bond attachment. Layered-composite insert  45  comprises a plurality of layers wherein the layers may include OFEcopper/aluminum nitride/OFEcopper, copper/molybdenum/copper, or even copper/graphite (Table 1). In a preferred embodiment, layered-composite insert  45  includes an intermediate layer  37  of molybdenum, a top layer  42  of copper and a bottom layer  43  of copper, and may be formed with a periphery that conforms or is complementary to the geometric “foot-print” of semiconductor device  30 , e.g., square, rectangular, circular or ellipsoidal, etc. When mounted, the surface of semiconductor device  30  only makes thermal contact with a top mounting surface  47  of layered-composite insert  45 . 
         [0053]    Referring to  FIGS. 9-12 , a capillary wick  33  may be formed on a surface of layered-composite insert  45 . Also, layered-composite insert  45  may be complementarily formed or machined so as to have a central prominence  48  projecting upwardly into opening  55 , thereby to improve engagement with annular ledge  60  and vertical wall  62  ( FIG. 11 ). In this way, the top or bottom surfaces of layered-composite insert  45  may be arranged in coplanar relation with a top or bottom surface of cold plate  50 . Of course, central prominence  48  may project beyond the top or bottom surfaces of any cold plate in order to form a land for engaging a semiconductor package. Also, wick  33  may be formed and arranged so as to overlie the entire outwardly facing surface of layered-composite insert  45  while only covering an adjacent potion of base  44 . Of course, wick  33  may be formed and arranged so as to overlie the entire surface of layered-composite insert  45  and base  44 . 
         [0054]    Referring to  FIG. 13 , a base  87  may include a plurality of layered-composite inserts  45  within a single high CTE cold plate  50 . Each low CTE layered-composite insert  45  may be joined to cold plate  50  in any one, or a combination of the foregoing fixation methods. 
         [0055]    Referring to  FIGS. 14-22  a planar heat pipe  100  may be formed in accordance with the present invention having one or more walls that comprise at least one of a copper/molybdenum/copper or copper/aluminum nitride/copper layered-composite substantially similar in structure to that of layered-composite portion  45 . For example, a planar heat pipe  100  may include a first plate  105  and a second plate  110  that are hermetically sealed at their respective peripheral edges so as to define a vapor chamber  112 . Vapor chamber  112  is partially evacuated and back filled with a suitable two-phase working fluid, e.g., water, Freon, ammonia, etc. A wick  120  is disposed upon one or more of the surfaces of the internally facing walls that together define vapor chamber  112 . 
         [0056]    In another embodiment, planar heat pipe  130  may be formed so as to include one or more layered-composite inserts  45  ( FIGS. 23-28 ). Either first plate  105  or second plate  110  may define one or more openings that are closed by the introduction of layered-composite inserts  45 . 
         [0057]    Referring to  FIGS. 29-30 , a heat transfer base  135  comprises a first plate  140  and a second plate  143  arranged to form a planar heat pipe. One or more openings in first plate  140  are hermetically sealed by the introduction of a layered-composite insert  45 . Each opening in first plate  140  is formed within first plate  140  by a piercing or forming process so as to form an outwardly projecting, annular wall  147 . In one example, high CTE cold plate  135  comprises a copper sheet that has been pierced so as to draw an outwardly projecting, substantially annular wall  147  defining an outwardly facing, annular surface  150 . The peripheral top or bottom surface of layered-composite insert  45  is arranged so as to engage annular surface  150  of annular wall  147 , and the two are fixedly bonded to one another by any of the aforementioned conventional techniques, such as brazing, soldering, adhesives, or direct bond attachment. Wick  33  may be formed within the closed recess in cold plate  135  that is defined by layered-composite  45  and annular wall  147 . 
         [0058]    It is to be further understood that the present invention is by no means limited only to the particular constructions herein disclosed and shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims.