Abstract:
A thermal interface material ( 40 ) includes a macromolecular material ( 32 ), and a plurality of carbon nanotubes ( 22 ) embedded in the macromolecular material uniformly. The thermal interface material includes a first surface ( 42 ) and an opposite second surface ( 44 ). Each carbon nanotube is open at both ends thereof, and extends from the first surface to the second surface of the thermal interface material. A method for manufacturing the thermal interface material includes the steps of: (a) forming an array of carbon nanotubes on a substrate; (b) submerging the carbon nanotubes in a liquid macromolecular material; (c) solidifying the liquid macromolecular material; and (d) cutting the solidified liquid macromolecular material to obtain the thermal interface material with the carbon nanotubes secured therein.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to thermal interface materials, and more particularly to a thermal interface material which conducts heat by using carbon nanotubes. 
     2. Description of Related Art 
     Electronic components such as semiconductor chips are becoming progressively smaller, while at the same time heat dissipation requirements thereof are increasing. Commonly, a thermal interface material is utilized between the electronic component and a heat sink in order to efficiently dissipate heat generated by the electronic component. 
     A conventional thermal interface material is made by diffusing particles with a high heat conduction coefficient in a base material. The particles can be made of graphite, boron nitride, silicon oxide, alumina, silver, or other metals. However, a heat conduction coefficient of the thermal interface material is now considered to be too low for many contemporary applications, because it cannot adequately meet the heat dissipation requirements of modem electronic components. 
     A new kind of thermal interface material has recently been developed. The thermal interface material is obtained by fixing carbon fibers with a polymer. The carbon fibers are distributed directionally, and each carbon fiber can provide a heat conduction path. A heat conduction coefficient of this kind of thermal interface material is relatively high. However, the heat conduction coefficient of the thermal interface material is inversely proportional to a thickness thereof, and the thickness is required to be greater than 40 micrometers. In other words, the heat conduction coefficient is limited to a certain value corresponding to a thickness of 40 micrometers. The value of the heat conduction coefficient cannot be increased, because the thickness cannot be reduced. 
     An article entitled “Unusually High Thermal Conductivity of Carbon Nanotubes” and authored by Savas Berber (page 4613, Vol. 84, Physical Review Letters 2000) discloses that a heat conduction coefficient of a carbon nanotube can be 6600 W/mK (watts/milliKelvin) at room temperature. 
     U.S. Pat. No. 6,407,922 discloses another kind of thermal interface material. The thermal interface material is formed by injection molding, and has a plurality of carbon nanotubes incorporated in a matrix material. A first surface of the thermal interface material engages with an electronic device, and a second surface of the thermal interface material engages with a heat sink. The second surface has a larger area than the first surface, so that heat can be uniformly spread over the larger second surface. 
     However, the thermal interface material formed by injection molding is relatively thick. This increases a bulk of the thermal interface material and reduces its flexibility. Furthermore, the carbon nanotubes are disposed in the matrix material randomly and multidirectionally. This means that heat does not necessarily spread uniformly through the thermal interface material. In addition, the heat does not necessarily spread directly from the first surface engaged with the electronic device to the second surface engaged with the heat sink. 
     A new thermal interface material which overcomes the above-mentioned problems is desired. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a thermal interface material having a reduced thickness, small thermal interface resistance, good flexibility and excellent heat conduction. 
     To achieve the above-mentioned object, the present invention provides a thermal interface material comprising macromolecular material and a plurality of carbon nanotubes embedded in the macromolecular material uniformly. The thermal interface material also comprises a first surface and an opposite second surface. Each carbon nanotube is open at two ends thereof, and extends from the first surface to the second surface of the thermal interface material. 
     Unlike in a conventional thermal interface material, the carbon nanotubes of the thermal interface material of the present invention are disposed in the macromolecular material uniformly and directionally. Thus, each carbon nanotube of the thermal interface material can provide a heat conduction path in a direction perpendicular to a main heat absorbing surface of the thermal interface material. This ensures that the thermal interface material has a high heat conduction coefficient. Furthermore, the thickness of the thermal interface material of the present invention can be controlled by cutting the macromolecular material. This further enhances the heat conducting efficiency of the thermal interface material and reduces the volume and weight of the thermal interface material. Moreover, each carbon nanotube is open at two ends thereof, and extends from the first surface to the second surface of the thermal interface material. This ensures the carbon nanotubes can contact an electronic device and a heat sink directly. Thus, the thermal interface resistance between the carbon nanotubes and the electronic device is reduced, and the thermal interface resistance between the carbon nanotubes and the heat sink is reduced. Therefore, the heat conducting efficiency of the thermal interface material is further enhanced. 
     Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side elevation of a substrate having a catalyst film attached thereon according to the present invention; 
         FIG. 2  is similar to  FIG. 1 , but showing an array of carbon nanotubes directionally formed on the substrate; 
         FIG. 3  is similar to  FIG. 2 , but showing the substrate with the carbon nanotubes immersed in a liquid macromolecular material; 
         FIG. 4  is similar to  FIG. 3 , but showing only the substrate, with the carbon nanotubes on the substrate embedded in solidified macromolecular material; 
         FIG. 5  is similar to  FIG. 4 , but showing only the solidified macromolecular material with the carbon nanotubes embedded therein after the solidified macromolecular material has been cut; that is, the thermal interface material of the present invention; and 
         FIG. 6  is similar to  FIG. 5 , but showing the thermal interface material sandwiched between an electronic device and a heat sink. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a substrate  11  having a catalyst film  12  attached thereon is shown. In  FIG. 2 , an array of carbon nanotubes  22  directionally formed on the substrate  11  is shown. The carbon nanotubes  22  are manufactured by way of thermal chemical vapor deposition or plasma enhanced chemical vapor deposition. In a preferred method of the present invention, the carbon nanotubes  22  are manufactured as follows. Firstly, the substrate  11  is provided. The substrate  11  can be made of glass, quartz, silicon, or alumina. In the preferred embodiment, the substrate  11  is made of porous silicon. A surface of the porous silicon is a porous layer. Diameters of apertures in the porous layer are extremely small, generally about 3 nanometers. Then the catalyst film  12  is uniformly disposed on the substrate  11  by chemical vapor deposition, thermal disposition, electron-beam disposition, or sputtering. The catalyst film  12  can be made of iron (Fe), cobalt (Co), nickel (Ni), or an alloy thereof. In the preferred embodiment, the catalyst film  12  is made of iron. 
     Secondly, the catalyst film  12  is oxidized to obtain catalyst particles (not shown). Then, the substrate  11  with the catalyst particles disposed thereon is placed in a reaction furnace (not shown), and a carbon source gas is provided in the reaction furnace at a temperature of 700-1000° C. to grow the array of carbon nanotubes  22 . The carbon source gas can be acetylene or ethene. A height of the array of carbon nanotubes  22  can be controlled by controlling the growth time thereof. Details of the method for growing the array of carbon nanotubes  22  can be found in pages 512-514, Vol. 283, Science 1999, and in pages 11502-11503, Vol. 123, J. Am. Chem. Soc. 2001. Moreover, U.S. Pat. No. 6,350,488 discloses a method for mass synthesis of arrays of carbon nanotubes. These three publications are incorporated herein by reference. 
       FIG. 3  shows the carbon nanotubes  22  with the substrate  11  immersed in a container  30  of liquid macromolecular material  32 . That is, after the growth of the carbon nanotubes  22  is completed, the liquid macromolecular material  32  is provided in order to completely immerse the carbon nanotubes  22  therewithin. The liquid macromolecular material  32  is selected from the group consisting of resin, silicone rubber, and rubber. In the preferred embodiment, the liquid macromolecular material  32  is silicone rubber. A viscosity of the liquid macromolecular material  32  is required to be below 200 cps (centipoise). 
     Referring to  FIGS. 4 and 5 , the substrate  11  having the the carbon nanotubes  22  immersed in the liquid macromolecular material  32  is taken out of the container  30 . Then, the liquid macromolecular material  32  is cooled and solidified. The solidified macromolecular material  32  with the carbon nanotubes  22  secured therein is peeled off from the substrate  11 , and is immersed in liquid paraffin. Then, the liquid paraffin is cooled and solidified. The solidified paraffin has high rigidity, which ensures that the solidified macromolecular material  32  when cut has highly even surfaces. A top portion of the solidified macromolecular material  32  is cut by a cutter (not shown) in a direction perpendicular to the long axes of the carbon nanotubes  22  and at a predetermined elevation thereof. Then a bottom portion of the solidified macromolecular material  32  is cut by the cutter in the same direction at a predetermined elevation. Finally, the solidified paraffin is removed by a suitable solvent such as xylene, to thereby obtain a thermal interface material  40 . 
     In alternative methods, the liquid macromolecular material  32  can be cooled and solidified while the substrate  11  having the carbon nanotubes  22  remains in the container  30 . Then the substrate  11  having the carbon nanotubes  22  secured in the solidified macromolecular material  32  is taken out of the container  30 . Further, the solidified macromolecular material  32  with the carbon nanotubes  22  secured therein can be peeled off from the substrate  11  after the cutting of the top portion of the solidified macromolecular material  32 . In such case, the solidified macromolecular material  32  with the carbon nanotubes  22  secured therein is immersed in liquid paraffin twice: once before the cutting of the top portion of the solidified macromolecular material  32 , and a second time before the cutting of the bottom portion of the solidified macromolecular material  32 . 
     Detailedly, the cutting process is performed as follows. Firstly, the top portion of the solidified macromolecular material  32  is cut by the cutter in the direction perpendicular to the long axes of the carbon nanotubes  22 . This removes the solidified macromolecular material  32  upon the carbon nanotubes  22 , so that each carbon nanotube  22  is open at a top end thereof. Secondly, the bottom portion of the cut macromolecular material  32  is cut by the cutter in the same direction in order that the thermal interface material  40  has a predetermined thickness. Thus, each carbon nanotube  22  is open at a bottom end thereof. The thickness of the thermal interface material  40  is preferably in the range from 1 to 1000 micrometers. In the preferred embodiment, the thickness of the thermal interface material  40  is 20 micrometers. 
       FIG. 5  shows the thermal interface material  40  of the present invention in isolation. The thermal interface material  40  comprises the solidified macromolecular material  32 , and the array of carbon nanotubes  22  embedded in the solidified macromolecular material  32  uniformly. The thermal interface material  40  has a first surface  42 , and a second surface  44  opposite to the first surface  42 . The carbon nanotubes  22  are substantially parallel to each other, and extend from the first surface  42  to the second surface  44 . That is, the carbon nanotubes  22  are perpendicular to the first surface  42  and the second surface  44 . Thus, each carbon nanotube  22  can provide a heat conduction path in a direction perpendicular to a selected main heat absorbing surface of the thermal interface material  40 . Therefore, the thermal interface material  40  has a high heat conduction coefficient and can conduct heat uniformly. 
       FIG. 6  shows an application of the thermal interface material  40  of the present invention. The thermal interface material  40  is sandwiched between a heat sink  60  and an electronic device  80 , to provide good heat contact between the heat sink  60  and the electronic device  80 . The first surface  42  of the thermal interface material  40  engages with a surface (not labeled) of the electronic device  80 , and the second surface  44  of the thermal interface material  40  engages with a surface (not labeled) of the heat sink  60 . 
     Because solidified paraffin is used in the above-described cutting process, this ensures that the first and second surfaces  42 ,  44  of the thermal interface material  40  are highly even. In addition, because the thickness of the thermal interface material  40  is on a micron scale, the thermal interface material  40  has good flexibility. Thus, even if the surface of the electronic device  80  is uneven, the thermal interface material  40  can provide good heat contact between the heat sink  60  and the electrical device  80 . Furthermore, each carbon nanotube  22  is open at both ends thereof, and extends from the first surface  42  to the second surface  44  of the thermal interface material  40 . This ensures that the carbon nanotubes  22  contact the electronic device  80  and the heat sink  60  directly. Thus the thermal interface resistance between the carbon nanotubes  22  and the electronic device  80  is reduced, and the thermal interface resistance between the carbon nanotubes  22  and the heat sink  60  is reduced. Therefore, the heat conducting efficiency of the thermal interface material  40  is further enhanced. 
     It is understood that the above-described embodiments and methods are intended to illustrate rather than limit the invention. Variations may be made to the embodiments and methods without departing from the spirit of the invention. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.