Abstract:
The invention relates to a method and apparatus for producing aligned carbon nanotube thermal interface structures using batch and continuous manufacturing processes. In a batch process a capacitor is immersed in a bath containing a slurry of thermoplastic polymer containing randomly oriented carbon nanotubes and energized to create an electrical field to orient the carbon nanotubes prior to curing. In a continuous process, slurry carried by a conveyor receives the nanotube aligning electric field from capacitors positioned on both sides of the conveyor bearing the slurry.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to providing cooling solutions to electronic circuits, and, more specifically, to methods and apparatus for the fabrication of a thermal interface structure using carbon nanotubes to improve thermal performance of a die containing an electronic circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the manner in which the embodiments of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention that are not necessarily drawn to scale and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  is a perspective view of an embodiment of a thermal interface structure manufactured in accordance with the present invention; 
         FIG. 2  is a is a process flow chart of the present invention; 
         FIG. 3  is an illustrative elevational schematic of apparatus for fabricating thermal interface structures including a vat containing a monodisperse slurry of polymer and nanotubes and movable and adjustable capacitors positioned for insertion into the slurry; according to an embodiment of the present invention; 
         FIGS. 4 through 9  are illustrative elevational schematics of a typical capacitor, in the apparatus depicted in  FIG. 1 , at various stages of manufacture in accordance with an embodiment of a manufacturing process; 
         FIG. 10  is a process flow chart of the embodiment of the invention shown in  FIGS. 3 through 9 ; 
         FIG. 11  is an illustrative elevational schematic of an a different apparatus for manufacturing thermal interface structures according to another embodiment of the invention; and 
         FIG. 12  is a process flow chart of the embodiment of the invention shown in FIG.  11 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a process of forming a thermal interface structure having aligned carbon nanotubes embedded in a polymer interstitial material. More specifically, it relates to processes for aligning carbon nanotube fibers suspended in a slurry of nanotubes and liquid polymer and curing the aligned composite to form a billet which can be formed into a thermal interface structure. The use of aligned carbon nanotube fibers in the thermal interface structure provides a thermal interface structure having high thermal conductivity. The thermal interface structure may be used, for example to provide a highly thermally conductive path between a surface of an electronic circuit and a surface of a cooling solution such as a heat sink. 
     Arrays of nano tubes are being currently manufactured by Nano-Lab, Inc. using a high temperature chemical vapor deposition process. Such arrays are manufactured on a one at a time basis at a high temperature which makes deposition of the nanotubes directly on a processor die unfeasible. The nanotubes in the arrays are primarily multi-walled and therefore do not have the purity and exceptional thermal conductivity of single-walled nanotubes. It is desired to provide a batch type or continuing manufacturing process for thermal interface structures which allows for control of the quality of the polymer and the nanotubes. 
       FIG. 1  is a perspective view of a thermal interface structure  10  formed in accordance with the present invention. Thermal interface structure  10  has a length L and a width W and a thickness t as shown in FIG.  1 . In practice, the length L and width W of thermal interface structure  10  are selected to provide a substantial heat exchange surface while falling within the outlines of the exposed surface of the electronic circuit such as a semiconductor die which is to be cooled. In one embodiment the length and width are 2 cm and 1 cm. Although in the present embodiment the structure  10  is shown in a highly regular form, for the purposes of illustration, it need not necessarily be as regular as shown. 
     The thickness t of the thermal interface structure  10 , in one embodiment, may be limited by the length of the carbon nanotubes available. In one embodiment it may fall within a range of about 5 to 20 microns. Single-walled nanotubes manufactured using varied processes are available. Such nanotubes may be manufactured having varying lengths. Of course, increasing the length of the nanotubes and the thickness of the thermal interface structure  10  will increase the thermal impedance of the path between the die and the heat sink. Because of the exceptionally high thermal conductivity of single-walled carbon nanotubes, however, there is little to be gained by attempting to limit the thickness of the thermal interface structure to a thickness less than the range of lengths of the particular microtubes that are available using currently available manufacturing processes. 
       FIG. 2  is a flow chart of an embodiment of a process of the present invention. The process begins in block  20  with preparing a slurry of a polymeric interstitial material and a quantity of randomly oriented carbon nanotubes. In block  22  an electrical field is applied to the slurry to align the carbon nanotubes with the direction of the electrical field. After aligning the carbon nanotubes, the slurry is cured in block  24  and the resulting billet of cured material is cut or otherwise formed into completed thermal interface structures  10  in block  26  by cutting or similar forming processes. In one embodiment, the resulting billet is used as thermal interface structure  10  without further cutting or other forming. 
       FIG. 3  illustrates apparatus according to an embodiment of the manufacturing process of  FIG. 2  for making the thermal interface structure  10  of the present invention. A vat  32 , the walls  33  of which are illustrated in cross-section, is filled with a composite monodispersed slurry  34  which is comprised of interstitial material  36  in liquid form and a plurality of single walled carbon nanotubes  38 . The carbon nanotubes  38  can be produced in accordance with a number of manufacturing processes and then formed into thermal interface structures  10  in accordance with embodiments of the present invention. The present invention provides a way to orient nanotubes  38  for optimal thermal conductivity characteristics of the resulting thermal interface structure  10 . Some embodiments of the present invention do not require operations to be carried out at the high temperatures inherent in chemical vapor deposition processes which are necessary to formation of aligned nanotubes directly on substrate surfaces. Batch or continuous processes in accordance with the present invention provide advantages over the production of thermal interface structures by forming aligned nanotubes directly on substrates. 
     In one embodiment, the interstitial material  36  is a polymer. In one embodiment polymer  36  is selected from the group of thermoplastic polymers selected from the group consisting of polycarbonate, polypropylene, polyacetal, polyoxymethylene and polyformaldehyde. Other suitable thermoplastic polymers can also be used. 
     Slurry  34  contains nanotubes  38  in monodisperse form, that is to say in a form having the lowest and narrowest possible dimensional scatter about a given nanotube length. Merely providing nanotubes  38  in a randomly oriented form in the monodisperse slurry  34  does not provide for optimal thermal conductivity characteristics in the polymeric matrix material  34 . Accordingly, it is necessary to provide for the orienting of nanotubes  38  in the interstitial material  36  prior to curing the slurry  34  into a billet  10  of thermal interface material. 
     The apparatus shown in  FIG. 3  provides for such orientation of the carbon nanotubes  38  in the thermal interface material. In the batch forming process apparatus shown in  FIG. 3 , vat  32  has a plurality of capacitors  40  associated with it for carrying out a batch process as illustrated in  FIGS. 4 through 9  for forming thermal interface structures  10  having a preferred thermal path defined by aligned nanotube fibers. 
     Capacitors  40  are, in the embodiment illustrated, parallel plate capacitors with pairs of plates  42 ,  44 . Each capacitor is, in the embodiment shown in  FIG. 3 , suspended from a movable transport mechanism  46  (shown simply as a rod in  FIG. 3 ) for lowering each capacitor  40  into the slurry  34  of interstitial polymer  36  and randomly aligned nanotubes  38 . Capacitor plates  42  and  44  are sized with their surface area dimensions selected so that one or more thermal interface structures  10  having width W and length L can be formed between a pair of plates. In addition to being movable vertically into and out of the slurry  34  in vat  32 , plates  42  and  44  are also adjustable toward and away from each other while maintaining their parallel orientation relative to each other. 
     In accordance with a first portion of the manufacturing process,  FIG. 4  shows, in a side elevation view, the edges of capacitor plates  42  and  44  being moved in a direction aligned with arrow  26  into slurry  34 . After insertion of the plates  42  and  44  of capacitor  40  into the slurry in  FIG. 4 , plates  42  and  44  are adjusted relative to each other to move them toward each other in the directions shown by arrows  50  and  52  respectively of FIG.  5 . It will be understood that relative movement of the plates toward each other by mechanism  46  may be achieved by moving of one or both of the plates  42 ,  44  toward each other. It is understood that it is a matter of design choice whether one or both of the plates are actually moved. 
     Once the plates  42  and  44  are adjusted to the desired spacing between them to provide a particular film thickness, the plates are withdrawn from the bath as shown in  FIG. 6. A  charge of the slurry  34  remains between the plates  42  and  44  of capacitor  40  due to the effect of surface tension of the liquid polymer interstitial material  36 . After the plates  42 ,  46  are removed from the slurry  34  in the vat  32 , plates  42  and  44  are connected to an appropriate voltage source to apply an electrostatic electric field between the plates as shown schematically in  FIG. 7  by the positive and negative polarity symbols  72  and  74 . The electrostatic field has the effect of aligning the carbon nanotubes in the slurry between the plates. The orientation of the aligned nanotubes is the same as the electric field so that the nanotubes are substantially perpendicular to capacitor plates  42  and  44 . 
     While the electric field is being applied and the nanotubes come into alignment, plates  42  and  44  may be brought closer together, as shown in  FIG. 8 , to squeeze out excess polymer  36  before commencing a curing phase which is commenced, in one embodiment, while the field is still being applied to assure that the nanotubes  38  remain properly oriented as curing commences. Curing is carried out, in one embodiment, by applying ultraviolet illumination to the composite material. In another embodiment, it is commenced by spraying a curing fluid on the material. In one embodiment, the curing is commenced while the field continues to be applied. In another embodiment, the field can be removed and curing thereafter commenced by relying upon the fact that the slurry has sufficient viscosity to hold the aligned nanotubes in an aligned orientation for a sufficient time period to carry out the curing to permanently hold the nanotubes in alignment. 
     At the completion of the curing phase, plates  42  and  44  are adjusted to open the gap between them to allow access to the billet of material which can in one embodiment be utilized in the form it is when removed or, in another embodiment, can be cut into a thermal interface structure  10  having the desired shape and dimensions. 
       FIG. 10  is a flow chart showing the process carried out by the embodiment illustrated in  FIGS. 3 through 9 . In block  1010  a slurry of carbon nanotubes and polymer is prepared. In block  1020  one or more capacitors are inserted into the slurry and the spacing of the plates of the capacitor is adjusted in block  1030 . The adjusted capacitor is removed from the slurry with a charge of slurry in block  1040  and an electrostatic electrical field is applied across the plates of the capacitor in block  1050 . The slurry with aligned carbon nanotubes is then cured in block  1060  and the cured billet is removed from between the plates of the capacitor in block  1070 . Finally the billet is, in one embodiment, formed into a thermal interface structure in block  1080 . In another embodiment, the billet can be used as the thermal interface structure without further forming. 
       FIG. 11  illustrates a different machine  1100 , used in a continuous forming process according to a further embodiment of the invention. A hopper  1192  is loaded with a slurry  34  which is comprised of a monodisperse of carbon nanotubes  38  in an interstitial material  36  which is generally the same as the one loaded in vat  32  in the batch manufacturing embodiment shown in FIG.  3 . Slurry  34  is dispensed from hopper  1192  through a control valve  1194  which acts in coordination with the movement of a conveyor  1196  to assure delivery of a layer of slurry  34  on conveyor  1196 . As conveyor  1196  passes through a pair of plates  1197  and  1198  of a capacitor  1199  which are aligned and mounted above and below conveyor  1196 , the spacing of the plates  1197  and  1198 , in one embodiment, controls the thickness of the material. Plates  1197  and  1198  are connected to a voltage source  1200  to provide an electrostatic electrical field between the plates which forces the carbon nanotubes  38  into alignment with it. The material with its aligned nanotubes is transported by the conveyor  1196  from the plates  1197  and  1198  of capacitor  1199  to a curing station  1202 . 
     In one embodiment, curing station  1202  is a curing lamp, such as an ultraviolet lamp which solidifies the polymeric material  36  of slurry  34 . In another embodiment curing station  1202  delivers a chemical spray which hardens polymer  36 . In both curing embodiments the degree to which the polymer is cured may be varied to provide, in one embodiment, a soft polymer which is advantageous for use as a thermal intermediate for mounting heat sinks to dies. In another embodiment, a harder polymer, which is more suitable to applications where the thermal intermediate is used under high pressure, further improves the heat transfer characteristics between the circuit or semiconductor die and the cooling solution or heat sink. 
       FIG. 12  shows the process carried out in the apparatus shown in FIG.  11 . Block  1210  is a preparation step where the slurry of carbon nanotubes and polymer is prepared in a manner similar to that shown for the embodiment described in  FIGS. 3 through 10 . The slurry is dispensed onto a conveyor in block  1220  and a field is applied between the plates of a capacitor bridging the conveyor and slurry in block  1230 . After aligning the carbon nanotubes the conveyor in block  1240  moves the material to a curing area where it is cured and then proceeds to block  1250  where the cured billet is formed into a thermal interface structure having the desired dimensions. 
     It will be readily understood to those skilled in the art that various other changes in the details, material, and arrangements of the parts and method stages which have been described and illustrated in order to explain the nature of this invention may be made without departing from the principles and scope of the invention as expressed in the following claims.