Patent Publication Number: US-2007121299-A1

Title: Heat transfer apparatus, cooled electronic module and methods of fabrication thereof employing thermally conductive composite fins

Description:
CROSS-REFERENCE TO RELATED PATENT/APPLICATIONS  
      This application contains subject matter which is related to the subject matter of the following patent and/or applications, each of which is assigned to the same assignee as this application and each of which is hereby incorporated herein by reference in its entirety:  
      “Electronic Device Cooling Assembly and Method Employing Elastic Support Material Holding a Plurality of Thermally Conductive Pins,” Campbell et al., Ser. No. 10/873,432, filed Jun. 22, 2004;  
      “Fluidic Cooling Systems and Methods for Electronic Components,” Pompeo et al., Ser. No. 10/904,555; filed Nov. 16, 2004;  
      “Cooling Apparatus, Cooled Electronic Module, and Methods of Fabrication Thereof Employing Thermally Conductive, Wire-Bonded Pin Fins,” Campbell et al., Ser. No. 11/009,935, filed Dec. 10, 2004;  
      “Cooling Apparatus, Cooled Electronic Module and Methods pf Fabrication Thereof Employing an Integrated Manifold and a Plurality of Thermally Conductive Fins”, Campbell et al., Ser. No. 11/124,064, filed May 6, 2005;  
      “Cooling Apparatus, Cooled Electronic Module and Methods of Fabrication Thereof Employing an Integrated Coolant Inlet and Outlet Manifold,” Campbell et al., Ser. No. 11/124,513, filed May 6, 2005; and  
      “Electronic Device Substrate Assembly With Multilayer Impermeable Barrier and Method of Making”, Chu et al., U.S. Pat. No. 6,940,712 B2, issued Sep. 6, 2005. 
    
    
     TECHNICAL FIELD  
      The present invention relates to heat transfer mechanisms, and more particularly, to heat transfer apparatuses, cooled electronic modules and methods of fabrication thereof for removing heat generated by one or more electronic devices. Still more particularly, the present invention relates to heat transfer apparatuses and methods employing a plurality of thermally conductive composite fins, for example, wire-bonded to a substantially planar main surface of a thermally conductive base, which comprises part of or is coupled to an electronic device to be cooled.  
     BACKGROUND OF THE INVENTION  
      As is known, operating electronic devices produce heat. This heat must be efficiently removed from the devices in order to maintain device junction temperatures within desirable limits, with failure to remove the heat thus produced resulting in increased device temperatures, potentially leading to thermal runaway conditions. Several trends in the electronics industry have combined to increase the importance of thermal management, including heat removal for electronic devices, including technologies where thermal management has traditionally been less of a concern, such as CMOS. In particular, the need for faster and more densely packed circuits has had a direct impact on the importance of thermal management. First, power dissipation, and therefore heat production, increases as device operating frequencies increase. Second, increased operating frequencies may be possible at lower device junction temperatures. Further, as more and more devices are packed onto a single chip, heat flux (Watts/cm 2 ) increases, resulting in the need to remove more power from a given size chip or module. These trends have combined to create applications where it is no longer desirable to remove heat from modern devices solely by traditional air cooling methods, such as by using air cooled heat sinks with heat pipes or vapor chambers. Such air cooling techniques are inherently limited in their ability to extract heat from an electronic device with high power density.  
      Thus, the need to cool current and future high heat load, high heat flux electronic devices, mandates the development of aggressive thermal management techniques, such as liquid cooling using finned cold plate devices. Various types of liquid coolants provide different cooling capabilities. In particular, fluids such as refrigerants or other dielectric liquids (e.g., fluorocarbon liquid) exhibit relatively poor thermal conductivity and specific heat properties, when compared to liquids such as water or other aqueous fluids. Dielectric liquids have an advantage, however, in that they may be placed in direct physical contact with electronic devices and interconnects without adverse affects such as corrosion or electrical short circuits. Other cooling liquids, such as water or other aqueous fluids, exhibit superior thermal conductivity and specific heat compared to dielectric fluids. Water-based coolants, however, must be kept from physical contact with electronic devices and interconnects, since corrosion and electrical short circuit problems are likely to result from such contact. Various methods have been disclosed in the art for using water-based coolants, while providing physical separation between the coolants and the electronic device(s). With liquid-based cooling apparatuses, however, it is still necessary to attach the cooling apparatus to the electronic device. This attachment results in a thermal interface resistance between the cooling apparatus and the electronic device. Thus, in addition to typical liquid cooling issues regarding sealing and clogging due to particulate contamination, other issues such as thermal conductivity of the cooling apparatus, effectiveness of the interface to the electronic device as well as the thermal expansion match between the cooling apparatus and the electronic device and manufacturability, need to be addressed.  
     SUMMARY OF THE INVENTION  
      The shortcomings of the prior art are overcome and additional advantages are provided through the provision of a heat transfer apparatus. The heat transfer apparatus includes a thermally conductive base having a main surface, and a plurality of thermally conductive fins extending from the main surface of the thermally conductive base and disposed to facilitate transfer of heat from the thermally conductive base. At least some fins of the plurality of thermally conductive fins are composite structures. Each composite structure includes a first material coated with a second material, wherein the first material has a first thermal conductivity and the second material has a second thermal conductivity.  
      In enhanced aspects, the second thermal conductivity of the second material coating the first material is greater than the first thermal conductivity of the first material. As specific examples, the first material and the second material can respectively comprise one of: copper and diamond, gold and copper or gold and diamond. Further, the plurality of thermally conductive fins may include a plurality of thermally conductive pin-fins, which are wire-bonded to the main surface of the thermally conductive base. The thermally conductive base may either comprise a portion of an electronic device to be cooled, or a separate structure coupled to the electronic device to be cooled.  
      In another aspect, a cooled electronic module is provided which includes a substrate with at least one heat generating electronic device attached thereto, and a heat transfer apparatus coupled to the at least one heat generating electronic device for facilitating cooling thereof. The heat transfer apparatus includes a plurality of thermally conductive fins extending from one surface of the at least one heat generating electronic device or a thermally conductive base coupled to a surface of the at least one heat generating electronic device. The plurality of thermally conductive fins are disposed to facilitate transfer of heat from the at least one heat generating electronic device, and at least some fins of the plurality of thermally conductive fins are composite structures. Each composite structure includes a first material coated with a second material, wherein the first material has a first thermal conductivity and the second material has a second thermal conductivity.  
      In a further aspect, a method of fabricating a heat transfer apparatus is provided. This method includes: providing a thermally conductive base having a main surface; providing a plurality of thermally conductive fins extending from the main surface of the thermally conductive base, wherein the plurality of thermally conductive fins are disposed across the main surface of the thermally conductive base to facilitate transfer of heat from the thermally conductive base; and coating at least some thermally conductive fins of the plurality of thermally conductive fins with a thermally conductive material to increase the thickness of each thermally conductive fin of the at least some thermally conductive fins and thereby facilitate transfer of heat from the thermally conductive base via the plurality of thermally conductive fins.  
      Further, additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:  
       FIG. 1  is a cross-sectional elevational view of one embodiment of a cooled electronic module, in accordance with an aspect of the present invention;  
       FIG. 2  is an isometric view of one embodiment of a cooling or heat transfer apparatus, in accordance with an aspect of the present invention;  
       FIG. 3A  is an elevational view of a pin-fin wire to be wire-bonded to a thermally conductive base during a cooling apparatus fabrication method, in accordance with an aspect of the present invention;  
       FIG. 3B  depicts the structures of  FIG. 3A  showing the formation of a diffusion weld-bond between the pin-fin wire and the thermally conductive base, in accordance with an aspect of the present invention;  
       FIG. 3C  depicts the structures of  FIG. 3B  showing the wire-bonding tool head in unclamped position being moved up the wire, in accordance with an aspect of the present invention;  
       FIG. 3D  depicts the structures of  FIG. 3C  with the wire-bonding tool head reclamped at a higher position along the wire that is to comprise the pin-fin, in accordance with an aspect of the present invention;  
       FIG. 3E  depicts the structures of  FIG. 3D  after bending of the wire and formation of another diffusion weld-bond with the thermally conductive base at a further point along the wire, in accordance with an aspect of the present invention;  
       FIG. 3F  depicts the structures of  FIG. 3E  showing the application of an electronic flame off (EFO) to the wire to cut the wire and thereby form the discrete, looped pin-fin, in accordance with an aspect of the present invention;  
       FIG. 3G  depicts the structures of  FIG. 3F  after the wire has been cut and the discrete, looped pin-fin formed, in accordance with an aspect of the present invention;  
       FIG. 4A  is an elevational view of one embodiment of a cooling or heat transfer apparatus formed using the fabrication method of  FIGS. 3A-3G , in accordance with an aspect of the present invention;  
       FIG. 4B  is an elevational view of the structure of  FIG. 4A  showing a pre-tinned manifold plate being brought down into physical contact with an upper surface of the discrete, looped pin-fins, in accordance with an aspect of the present invention;  
       FIG. 4C  is an elevational view of the structure of  FIG. 4B , after the application of heat to reflow solder and thereby physically connect the discrete, looped pin-fins and the manifold plate, in accordance with an aspect of the present invention;  
       FIG. 5  is an elevational view of an alternate embodiment of a cooling or heat transfer apparatus, in accordance with an aspect of the present invention;  
       FIG. 6  is a cross-sectional elevational view of an alternate embodiment of a cooled electronic module, in accordance with an aspect of the present invention;  
       FIG. 7  is a cross-sectional elevational view of another embodiment of a cooled electronic module, in accordance with an aspect of the present invention;  
       FIG. 8  is a cross-sectional elevational view of still another embodiment of a cooled electronic module, in accordance with an aspect of the present invention;  
       FIG. 9  is a cross-sectional elevational view of a further embodiment of a cooled electronic module, in accordance with an aspect of the present invention;  
       FIG. 10  is a partial plan view of one embodiment of a cooling or heat transfer apparatus, in accordance with an aspect of the present invention;  
       FIG. 11  is a graph illustrating thermal conductivity for various heat sink materials, one or more of which could be employed in a fin array of a cooling or heat transfer apparatus, in accordance with an aspect of the present invention;  
       FIG. 12  illustrates deposition of a thermally conductive material onto a plurality of thermally conductive pin-fins to form composite pin-fin structures, in accordance with an aspect of the present invention;  
       FIG. 13A  is a partial plan view of one embodiment of a cooling or heat transfer apparatus employing the composite pin-fin structures of  FIG. 12 , in accordance with an aspect of the present invention;  
       FIG. 13B  is an isometric view of one composite pin-fin structure of the array of composite pin-fin structures illustrated in  FIG. 13A , in accordance with an aspect of the present invention;  
       FIG. 14  is a graph of pin-fin efficiency versus pin-fin height for copper pin-fins compared with a composite copper-diamond pin-fin structure, in accordance with an aspect of the present invention; and  
       FIG. 15  is a graph of heat rate dissipation per pin-fin compared with pin-fin height, contrasting copper-only pin-fins with a composite copper-diamond pin-fin structure, in accordance with an aspect of the present invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      As used herein, “electronic device” comprises any heat generating electronic component of a computer system or other electronic system requiring cooling. In one example, the electronic device includes an integrated circuit chip. The term “cooled electronic module” includes any electronic module with cooling and at least one electronic device, with single chip modules and multichip modules being examples of electronic modules to be cooled. As used herein, “micro-scaled cooling structure” means a cooling structure with a characteristic dimension of 200 micrometers (microns) or less. A “composite” fin structure means any fin structure wherein a first material having a first thermal conductivity is coated or encapsulated by a second material having a second thermal conductivity. Each “material” may either be an element or a compound that is thermally conductive.  
      Generally stated, provided herein is an enhanced cooling apparatus and method of fabrication which allow for a high heat transfer rate from a surface of an electronic device to be cooled using a direct or indirect liquid coolant approach. In one embodiment, the cooling liquid may comprise a water-based fluid, and the cooling apparatus may be employed in combination with a passivated electronic substrate assembly. However, the concepts disclosed herein are readily adapted to use with other types of coolant. For example, the coolant may comprise a brine, a fluorocarbon liquid, a liquid metal, or other similar coolant, or a refrigerant, while still maintaining the advantages and unique features of the present invention.  
      One possible implementation of a micro-scaled cooling structure is a micro-channeled cold plate fabricated, e.g., of copper or silicon. A micro-channel copper cold plate has an advantage of having high thermal conductivity, and thus being effective in spreading heat for convective removal by a cooling liquid. However, copper has a much higher thermal expansion coefficient than silicon, which is typically employed in integrated circuit chips. The thermal expansion coefficient of copper is approximately eight times that of silicon. This difference in thermal expansion between copper and silicon prevents the use of an extremely thin (and thus thermally superior) interface between a micro-channeled copper cold plate and a silicon chip, and also prevents the use of relatively rigid interfaces such as solder or a thermally cured epoxy. Instead, such a copper cold plate would require the use of a thermal grease interface, which can be as much as two to three times higher in thermal resistance than solder or epoxy interfaces. Thus, although the thermal performance of a micro-channeled copper cold plate is excellent, it can not be placed in correspondingly excellent thermal contact with a conventional electronic device, thus diminishing the overall module thermal performance.  
      In an alternate implementation, a micro-channel cold plate could be fabricated of silicon, which can be bonded to a silicon chip via solder or thermally cured epoxy. However, the thermal conductivity of silicon is approximately one-third that of copper, thus making any micro-scaled, finned structure made of silicon less efficient in spreading heat for extraction by the liquid coolant.  
      Further, in a micro-channeled cold plate, channel dimensions can be exceedingly small, e.g., less than 65 micrometers, which heightens the risk of clogging by micro-particulate contamination over the lifetime of the cooling apparatus. Also, due to the small channel dimensions in a micro-channel heat sink, the pressure drop through such a cooling apparatus can be prohibitively high. A goal of the present invention, therefore, is to alleviate the clogging and pressure drop drawbacks, as well as the drawbacks found in the above described copper and silicon micro-channeled cold plates, while still displaying excellent thermal performance necessary to cool high performance heat flux electronic devices.  
      Reference is now made to the drawings, wherein the same reference numbers used throughout different figures designate the same or similar components.  FIG. 1  depicts one embodiment of a cooled electronic module, generally denoted  100 , in accordance with an aspect of the present invention. In this embodiment, cooled electronic module  100  includes a substrate  110 , which may include conductive wiring (not shown) on an upper surface thereof and/or imbedded therein. An integrated circuit chip  120  is electrically connected to the wiring of substrate  110  via, for example, solder ball connections  125 . A sealing structure  130  facilitates isolation of the active circuit portion of the integrated circuit  120  from liquid coolant within the module. A base plate  140  covers integrated circuit chip  120  and a portion of the sealing structure  130 . A housing  170  is hermetically sealed  175  to base plate  140  and sealing structure  130  via, for example, soldering or brazing. Within the housing, a plurality of pin-fins  150  extend from base plate  140  into a coolant flow path defined by the housing. In one example, these pin-fins each comprise discrete, looped pin-fins fabricated of copper. The coolant flow path includes an inlet manifold  160  disposed above, and in the embodiment shown, contacting an upper surface of the plurality of pin-fins. Inlet manifold  160  includes an inlet  162  and a plurality of orifices  164 , which may comprise micro-scaled orifices. Housing  170  includes a liquid coolant outlet  172  for removal of coolant after flowing around the plurality of pin-fins  150  and the thermally conductive base  140 . Although the manifold scheme depicts central coolant inlets with peripheral outlets, a number of different schemes may be incorporated without departing from the scope of the present invention.  
       FIG. 2  depicts a perspective view of one embodiment of a micro-scaled cooling structure or apparatus in accordance with an aspect of the present invention. In this example, the structure comprises a cold plate having a thermally conductive base  140  with a substantially planar upper surface from which a plurality of discrete, looped pin-fins  150  project in an array. The looped pin-fins may comprise copper wire, and the thermally conductive base  140  a material of high thermal conductivity. Base  140  is assumed to have a coefficient of thermal expansion within a defined range of a coefficient of thermal expansion of the electronic device to be cooled, which may, e.g., comprise silicon. In one example, the defined range may be ±1.5×10 −6  1/K. Assuming that the electronic device comprises silicon, then the coefficient of thermal expansion of the thermally conductive base is preferably in a range of 0.9×10 −6  1/K. to 4.1×10 −6  1/K.  
      By way of specific example, the pin-fins may be 1-3 mm in height, and have diameters of about 50-250 micrometers, arranged with a pin-to-pin pitch in the 50-500 micrometer range. Thus, the cooling structure  200  of  FIG. 2  has the advantage of utilizing a first thermal conductivity material for the fins (e.g., copper), and a second thermal conductivity material for the base, which can be attached to a silicon chip with an excellent interface without concerns related to a coefficient of thermal expansion mismatch (which is a common problem with many previous cooling structures). By way of example, the thermally conductive base  140  could comprise silicon carbide, aluminum nitride, a copper-molybdenum-copper composite, diamond, silicon, etc. The cooling apparatus of  FIG. 2  has a large free area and a large free volume ratio, thus making the design significantly less susceptible to clogging over the lifetime of the product compared with a finned micro-channel cold plate such as described above. A simple manifold scheme is sufficient to ensure reliable low pressure drop operation for both single chip module and multichip module applications. Numerous variations to an inline geometry are also possible without departing from the scope of the present invention. Further, in embodiments discussed below, the thermally conductive base  140  could comprise, for example, a back surface of an integrated circuit chip. In addition, the dimensions and shapes of the pin-fins are preferably chosen to ensure a large free area and large free volume ratio to minimize susceptibility to clogging. In the event that thicker pin-fins are desired, the wire-bonded pin-fins can be electroplated to achieve a desired diameter.  
      In accordance with the present invention, the thermally conductive pin-fins are wire-bonded to a substantially planar main surface of the thermally conductive base  140 , and as noted, base  140  could comprise a portion of the electronic device to be cooled. For example, base  140  could comprise the integrated circuit chip. Different wire-bonding techniques can be employed to create a looped micro-pin-fin array such as depicted in  FIG. 2 . For example, ball wire-bonding and wedge wire-bonding could be employed, both of which are conventionally used for creating chip-to-substrate interconnections. Numerous wire-bonding machines are available in the art. For example, various ball and wedge wire-bonding machines are manufactured and available through Kulicke &amp; Soffa of Willow Grove, Pennsylvania.  
       FIGS. 3A-3G  illustrate one method for fabricating a cooling apparatus in accordance with the present invention using thermosonic ball-bonding techniques.  FIG. 3A  depicts the beginning of the manufacturing process, displaying the various elements needed for the process, including a thermally conductive base  140 , and a wire  300  that is to comprise the pin-fin. Wire  300  includes a ball tip  305  and the tool head that incorporates the wire clamping mechanism includes a capillary passage  310  for the wire. Appropriate metallization (such as chrome-copper or chrome-copper-gold) is assumed to reside on an upper surface of the thermally conductive base  140 . In  FIG. 3A , the clamping mechanism of the tool head  312  is shown in an unclamped position.  
       FIG. 3B  illustrates the tool head  312  in a clamped position with the motion of the tool head being such as to enable physical contact between the ball tip of wire  300  and the metalized surface of the thermally conductive base  140 . A controlled downward bond force is applied in combination with ultrasonic activation, and the two in combination create a physical environment that is conducive to plastic deformation and intermolecular diffusion between wire  300  and the metalized base. A diffusion weld-bond  315  results under these conditions, whereby the plastic deformation at microscopic length scales cause the metal to flow in slip and shear planes across each part of the wire-substrate interface, thus forming a metallurgical diffusion bond. After the bond is formed, the tool head is unclamped, as shown in  FIG. 3C , and moved to different position along the length of wire  300 , where the tool head  312  is again clamped, as shown in  FIG. 3D .  
       FIG. 3E  depicts the assembly of  FIG. 3D  after the wire has been bent back downward to contact the base  140  and the tool head has been used to form a second diffusion weld-bond  320 , thereby ending the pin-fin loop. This second bond  320  is a tail which is a result of the process.  FIG. 3F  shows the tool head removed along the wire  300  to a new position to allow space for an electronic flame off (EFO) operation, which is a process known in the art for cutting a wire. The electronic flame off operation severs wire  300  at the end of second diffusion weld-bond  320 , and also creates a new ball tip  305  to allow re-initiation of the process described above, as shown in  FIG. 3G . In  FIG. 3G , the tool head is in the ready position to repeat the steps illustrated in  FIGS. 3A-3F .  
      After numerous repetitions of the process described in  FIGS. 3A-3F , a micro-pin-fin array such as depicted in  FIG. 4A  can be created. In this array, a plurality of discrete, looped pin-fins  150  are closely spaced and diffusion weld-bonded to a thermally conductive base, which as noted above, can comprise part of the electronic device itself to be cooled, or can comprise a separate structure which has a coefficient of thermal expansion closely matched to that of the electronic device to be cooled.  FIG. 4B  shows a pre-tinned top manifold plate  400  that is brought down to be in slight pressurized contact with the tops of the looped pin-fins  150  as shown in  FIG. 4C . The manifold plate includes one or more inlet ports or orifices  410 , which in one embodiment, may comprise micro-scaled openings. Manifold plate  400  is then heated, for example, by placing the assembly in an oven or by other heating techniques, to reflow the solder and create a rigid joint  420  between the tops of the pin-fins  150  and manifold plate  400 . Such solder joints serve to increase the rigidity of the pin-fins, thus reducing any propensity of the fins to deform when subjected to high velocity cross-flow of a liquid coolant.  
       FIG. 5  depicts an alternate embodiment of a cooling apparatus in accordance with the present invention. In this embodiment, straight pin-fins  520  are shown extending from a substantially planar surface of a substrate  510 , which again is assumed to comprise a thermally conductive base. The pin-fins  520  are diffusion weld-bonded  525  to base  510 , for example, via thermosonic weld-bonding such as described above. Fabrication of this cooling apparatus can employ the process of  FIGS. 3A-3G , with the electronic flame-off operation described in  FIG. 3F  occurring earlier, for example, at the step depicted in  FIG. 3D .  
      Process cycle times for forming the diffusion bonds of  FIGS. 3A-5  are less than 20 milliseconds. Thus, to create a high performance pin-fin array such as depicted in  FIG. 2 , wherein 2500 pin-fins are employed to cool a surface of 1 cm 2  (and hence 2500 bonds), the bonding process time can be estimated to be about 50 seconds. This is a reasonable time for cost effective production of a single cooling apparatus such as described herein. Further, those skilled in the art will note that wire-bonding machines are advanced computer controlled machines that can be programmed to create non-uniform patterns of arrays that represent different embodiments of the design depicted in  FIGS. 2-5 .  
      As noted briefly, another technique which can be used to create enhanced heat transfer fin structures is a wedged bonding approach. The process times for wedge bonding, have been reported to be less than 80 milliseconds per bond, which again allows for a practical implementation of the concepts disclosed herein.  
      Advantageously, the structures described herein provide an excellent thermal interface due to the metallurgical nature of a wire-bond, and due to the absence of a third material, such as solder or braze compound, between the pin-fins and the base. The wire-bonding approach described is particularly beneficial when creating a silicon-to-copper pin bond, for example, for the discrete, looped micro-pin-fins. The pin-fin to substrate bonds are created using a wire bonding process that employs ultrasonic activation, and establishes a diffusion weld-bond between surfaces that are metallurgically clean, e.g., free of oxides, and which are highly energetic. These interface properties make for an excellent thermal interface of low thermal resistance.  FIGS. 3B-3G  illustrate the shape of the fin at its base, directly above the silicon-to-copper pin bond. This hemispherical shape allows for a larger surface area at the bond, approximately 2-4 times the diameter of the wire itself, thus significantly increasing the contact area, thereby reducing the interface/contact thermal resistance at the interface. The thermal interface resistance at these pin-to-base interfaces is inversely proportional to the area of contact. Additionally, the hemispherical diffusion weld-bond shape allows for “thermal merging” as the heat flows from the large cross-sectional area of the thermally conductive base, to the smaller cross-sectional area of the pin-fins, thereby reducing the constriction resistance of the fin structure to heat flow.  
       FIGS. 6-8  depict alternate embodiments of cooled electronic modules employing a cooling apparatus in accordance with the present invention. In  FIG. 6 , a substrate  610  again supports and is electrically connected to an electronic device  620  via a plurality of interconnects, such as solder ball connections  625 . Sealing structures  630  isolate the active componentry of device  620  from the cooling liquid flowing within housing  670 . Housing  670 , in this example, is sealed directly to the sealing structure  630  and creates a cavity within which an inlet manifold  660  is provided. Inlet manifold  660  includes an inlet  662  and one or more orifices  664  for directing cooling liquid onto a surface of the electronic device to be cooled  620 . A plurality of pin-fins  650  are shown interconnected between the electronic device  620  and the inlet manifold  660 . Again, pin-fins  650  may comprise discrete, looped pin-fins manufactured of copper in a manner similar to that described above in connection with  FIGS. 3A-4C . In this embodiment, however, the looped pin-fins are wire-bonded directly onto the surface of the electronic device, to thus create the fin structure for direct liquid cooling. Further, in this embodiment, the electronic device may be passivated from the liquid coolant via an impermeable barrier (not shown) such as described in commonly assigned U.S. Pat. No. 6,940,712 B2, issued Sep. 6, 2005, and entitled “Electronic Device Substrate Assembly With Multi-Layer Impermeable Barrier And Method of Making,” the entirety of which is hereby incorporated herein by reference.  
       FIGS. 7 &amp; 8  depict examples of cooled electronic modules which comprise multichip modules. In  FIG. 7 , the cooled electronic module  700  includes a substrate  710  supporting multiple electronic devices  720 , which in one example may comprise bare integrated circuit chips. Devices  720  are shown electrically interconnected via solder ball connections  725  to metallization on or embedded within the substrate  710  supporting the electronic devices. Appropriate sealing structures  730  facilitate sealing the electronic devices  720  from the liquid coolant. A thermally conductive base  740  is shown coupled to each electronic device. Each base  740  is assumed to comprise a thermally conductive base material which has a coefficient of thermal expansion within a defined range of the coefficient of thermal expansion of the respective electronic device to be cooled. As one example, the electronic device may comprise silicon, and the defined range may be ±1.5×10 −6  1/K from the coefficient of thermal expansion of silicon. As noted above, the thermally conductive base may comprise various materials, including, silicon carbide, aluminum nitride, diamond, a copper-molybdenum-copper composite, silicon, etc. A plurality of pin-fins  750  extend from a substantially planar surface of the thermally conductive base  740 . In one example, these pin-fins may comprise discrete, looped pin-fins such as those described above in connection with  FIGS. 3A-4C . An inlet plenum  760  rests on, and may be soldered or brazed to, the plurality of pin-fins  750 . Inlet plenum  760  includes a coolant inlet  762  and one or more orifices  764  disposed over respective cooling apparatuses  740 ,  750  coupled to the electronic devices  720 . Housing  770  is again sealed to the sealing structure  730  and defines an inner liquid coolant flow path through which liquid coolant flows from orifices  764  to one or more exits  772  in the housing  770 .  
       FIG. 8  depicts another alternate embodiment of a cooled electronic module  800 , which is again a multichip module, wherein pin-fins  850  are directly wire-bonded to the electronic devices  820 , such as integrated circuit chips. The electronic devices  820  are electrically connected  825  to a supporting substrate  810 , and a sealing structure  830  facilitates isolation of the active circuitry of the electronic devices. An appropriate liquid impermeable passivation layer (not shown) could reside atop the electronic devices depending upon the liquid coolant employed. The plurality of pin-fins  850  comprise (in one example) discrete, looped pin-fins fabricated of copper. These pin-fins are diffusion weld-bonded to the exposed surfaces of the electronic devices  820 . Housing  870  is a manifold structure which defines a liquid coolant flow path from an inlet  862  in an inlet plenum  860  through inlet orifices  864  to one or more coolant outlets  872 .  
      By way of further example, analysis was performed to characterize cooling for a silicon chip of 0.75 mm thickness and 1 cm 2  footprint area, with a micro-pin cooling apparatus as presented herein. The geometry modeled represented looped pin arrays with 2500 pins per square centimeter, each 1 mm tall, and 50 or 75 micrometers in diameter, and arranged orthogonally in two dimensions with a pitch of 100 micrometers and 200 micrometers, respectively. In a flow distribution similar to that illustrated in  FIGS. 6-8 , coolant entered from a center of the finned cooling structure and exited from the periphery. Water was utilized as the coolant, at a volumetric flow rate of 0.25 gallons per minute for the entire 1 cm 2  chip. The pin-fins were made of copper and the cooling apparatus was assumed to comprise a heat sink base of 125 micron silicon carbide that was soldered to a silicon chip. Results illustrate excellent thermal performance of 310-370 W/cm 2  with a chip to ambient temperature difference of 60° C., and relatively low pressure drops of between 1.2-1.5 psi for the two pin diameters of 50 and 75 micrometers, respectively.  
       FIG. 9  depicts a further alternate embodiment of a cooled electronic module  900 , which includes a substrate  910 , that may include conductive wiring (not shown) on an upper surface thereof and/or embedded therein. An electronic device  920  is electrically connected to substrate  910  via, for example, solder ball connections  925 . A sealing structure  930 , which could comprise a plate with a center opening, facilitates isolation of the active portion of the electronic device  920  (as well as connections  925  and the substrate surface metallurgy) from coolant within the module. A sealant  935 , such as epoxy, provides a fluid-tight seal between sealing structure  930  and electronic device  920 . This seal is desirable particularly if the coolant is aqueous in nature. The housing  970  includes an inlet plenum housing  940  and an outlet plenum housing  980 . Inlet plenum housing  940  includes an inlet plenum  945  which receives coolant through at least one inlet opening  942  and directs coolant through a plurality of orifices  960 , disposed in an orifice plate  950 , onto the surface to be cooled. In one embodiment, orifices  960  comprise jet orifices which provide an impinging jet flow onto the surface to be cooled. After impinging on the surface to be cooled, the coolant flows outward towards the periphery of the electronic device, where it turns upwards and exits through an outlet plenum  985  via at least one outlet port  972 .  
      In this embodiment, a heat sink structure  990  (e.g., a micro-scaled structure) is coupled to electronic device  920  via a thermal interface  992 . This interface may comprise silicone, epoxy, solder, etc. Heat since structure  990  comprises a thermally conductive base having a main surface with a plurality of thermally conductive fins  994  extending therefrom to facilitate transfer of heat from the base, and hence from electronic device  920 .  
       FIG. 10  is a partial plan view of one embodiment of a cooling or heat transfer apparatus, generally denoted  1000 , which includes a thermally conductive base  1010  having a planar main surface  1015  from which a plurality of thermally conductive pin-fins  1020  extend. Thermally conductive pin-fins  1020  may comprise, in one implementation, copper pin-fins which advantageously spread heat over a large surface area that is in good thermal communication with coolant which carries the heat away. Copper has a thermal conductivity of about 400 W/m-K. However, thermal optimization of such copper pin-fin micro-structures for anticipated module coolant flow conditions (˜0.5 gpm, 1-3 psi) show that the copper pin efficiency, i.e., the ability of the fin to effectively spread the heat, significantly degrades at fin heights greater than 1-2 mm for pin-fins having diameters in the range of 0.025-0.1 mm. Analysis has also indicated that further gains in thermal performance can be achieved from denser pin-fin arrays, with greater area coverage by the pin-fins. Further, chemically active coolants (e.g., Dynalene™ liquids offered by Dynalene Heat Transfer Fluids, of Whitehall, Pa.), may be advantageous as a liquid coolant. These fluids can potentially harm a copper micro-structure. Therefore, enhancements to a copper pin-fin micro-structure such as described above may be desirable depending upon the implementation.  
       FIG. 11  graphically illustrates comparison of thermal conductivity (W/m-K) for various materials. As shown, commercially available chemical vapor deposition (CVD) diamond, such as manufactured by Diamonex Products of Allentown, Pennsylvania, has a thermal conductivity of approximately 1300 W/m-K. There are many different processes for the creation of CVD diamond, such as plasma arcing, plasma discharge, hot filament, and combustion synthesis. Each of these processes uses some form of energy to break down hydrocarbons (CH 4 , etc.) to yield diamond (carbon). The CVD diamond manufactured by Diamonex Products is made by the thermal hot filament process, and is of the poly-crystalline form. There are several manufacturers of such machines that use the hot filament process to make CVD diamond, for example, SEKI Technotron Corporation of Tokyo, Japan.  
       FIG. 12  is a conceptual schematic of a CVD diamond deposition process for enhancing a heat transfer apparatus, generally denoted  1200 , comprising a thermally conductive base  1210  having a main surface  1215  with a plurality of thermally conductive pin-fins  1220  extending therefrom.  
      As noted, one method to create a composite pin-fin structure as described herein is the deposition of diamond on a metal, such as copper or gold. This can be accomplished by chemical vapor deposition (CVD) using the hot filament process noted above. All CVD diamond deposition processes involve the use of some form of energy to break down hydrocarbons such as methane (CH 4 ) to yield carbon (C). In the thermal hot filament process, heat is this form of energy. There are several manufactures of machines to create CVD diamond using this process, such as the machines made by SEKI Technotron Corporation of Tokyo, Japan. To make the structures disclosed herein, the wire-bonded pin-fin array is placed in a chamber that also houses the hot filament, and is exposed to deposition of CVD diamond (i.e., carbon) which is generated by a hot filament process. The temperature range for the substrate on which CVD diamond is deposited using the hot filament method is 700°-1000° C., and the pressure range is 10-100 torr. Typical deposition rates are between 0.3-40 microns of thickness/hour. The filament temperature is typically in the 2000°-2400° C. range.  
       FIG. 13A  is a partial plan view of the resultant heat transfer apparatus  1300  wherein each thermally conductive pin-fin  1320  is coated with a layer of CVD diamond  1330 , as more clearly depicted in  FIG. 13B .  
      As a specific example, pin-fins may be in the range of 0.025-0.1 mm in diameter, and be placed on a thermally conductive base with a center-to-center pitch in a range of 0.125-0.2 mm. The coating over the pin-fins may range in thickness from 0.025-0.05 mm. Thus, a 0.025 mm thick coating increases the diameter of 0.05 mm pin to be 0.1 mm.  
      Various heat transfer apparatus configuration are possible, with the pin-fin arrangement described herein being one example only. For example, plate fins could alternatively be employed extending from the main surface of the thermally conductive base, with each plate fin being coated as described herein with an enhanced thermally conductive material.  
       FIG. 14  illustrates the impact of pin-fin height on pin-fin efficiency for copper-only pin-fins compared with a composite copper-diamond pin-fin structure, such as disclosed herein. For both materials, the outside diameter of the pin-fin is 0.1 mm (˜4 mils.). Copper is assumed to have a thermal conductivity of 400 W/m-K, as noted in  FIG. 11 , and the value for copper-diamond is 1300 W/m-K, again, as illustrated in  FIG. 11 . A convective coefficient of 15000 W/m 2 -K is assumed to act over the outside fin surface, and this is commensurate with an anticipated flow condition for microprocessor module cooling, i.e., about 0.5 gallons per minute (gpm) flow and 1-3 pounds per square inch (psi) pressure drop. For the composite pin-fin, the original pin is 0.05 mm in diameter (˜2 mils), thus a 0.025 mm (˜1 mil) thick coating has been applied on the original pin. The results show in  FIG. 14  that the efficiency of the composite copper-diamond pin-fin degrades much less with fin height than the copper-only pin-fin.  
       FIG. 15  illustrates a comparison of heat transfer rates for increasing pin-fin heights for pure copper and composite copper-diamond pin-fins. For the conditions and geometry described above, the composite pin-fin out performs the pure copper fin by as much as 51% for 2 mm tall fins. For a fin-base to ambient-coolant temperature difference to 25° C., this translates to 300 W/cm 2  for the composite copper-diamond pin-fin array of 4 mil diameter pins at 8.5 mil pitch. A comparable copper-only pin-fin array would dissipate 200 W/cm 2 .  
      Those skilled in the art will note that the composite copper-diamond pin-fin structure described herein is presented by way of example only. Broadly stated, the present invention, in one aspect, is a heat transfer apparatus which includes a thermally conductive base having a main surface and a plurality of thermally conductive fins extending from the main surface of the base. The thermally conductive fins are disposed to facilitate transfer of heat from the base. At least some fins of the plurality of thermally conductive fins are composite structures, each comprising a first material coated with a second material, wherein the first material has a first thermal conductivity and the second material has a second thermal conductivity. In most implementations, the second thermal conductivity of the coating will be greater than the thermal conductivity of the first material.  
      By way of example, the first material and the second material could respectively comprise one of: copper and diamond, gold and copper or gold and diamond. Alternatively, the first material and the second material could comprise the same material, for example, copper. Such a structure may advantageously result from a coating process such as described herein, wherein copper pin-fins are coated with a layer of copper in order to increase the thickness, i.e., diameter, of the pin-fin to a size and density greater than current wire-bonding techniques allow. By way of example, current technology allows 2 mil diameter wire to be wire-bonded on a 6 mil pitch array. By then coating the 2 mil wire, for example, with a 1 mil coating, a composite pin-fin structure of 4 mils is achieved on a 6 mil array. This provides better convective heat transfer characteristics than possible with 2 mil wire on a 6 mil pitch. Further, by growing the geometry as proposed herein, pin-fin heights may be increased, and convective behavior improved between the heat transfer apparatus and liquid coolant flowing around the plurality of thermally conductive pin-fins.  
      Those skilled in the art will also note from the above discussion, that provided herein is a heat transfer apparatus, cooled electronic module and method of fabrication which advantageously provides: (i) an ability to improve thermal performance for the same height, same pin diameter, and a similar pressure drop through the heat transfer apparatus; (ii) an ability to increase the micro-structure fin height and thus the thermal performance, without suffering from loss of fin efficiency; (iii) an ability to improve manufactured pin-fin arrays to a much smaller pitch for the same pin diameter, and thus achieve higher heat transfer rates (simply increasing the density independently would increase the pressure drop, but when combined with taller fins, the pressure drop can be designed to be comparable); (iv) a diamond coating (in certain embodiments) which is chemically resistant to acids and alkalis - acidic coolants may advantageously be employed in a liquid cooling system due to their anti-freeze properties; and (v) an ability to selectively deposit an ultra-high thermal conductivity CVD film to locally improve the thermal performance of the micro-structure, thus addressing a chip or device hot spot problem.  
      Although preferred embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.