Abstract:
A thermal interface material ( 40 ) includes a silver colloid base ( 32 ), and an array of carbon nanotubes ( 22 ) disposed in the silver colloid base uniformly. The carbon nanotubes have nanometer-scale silver filled therein, are substantially parallel to each other, and extend from a first surface ( 42 ) to a second surface ( 44 ) of the thermal interface material. A method for manufacturing the thermal interface material includes the steps of: (a) forming an array of carbon nanotubes with nanometer-scale silver filled therein on a substrate; (b) submerging the carbon nanotubes in a silver colloid base; (c) solidifying the silver colloid base; and (d) peeling the solidified silver colloid base with the carbon nanotubes secured therein off from the substrate.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates generally to thermal interface materials and manufacturing methods thereof; and more particularly to a kind of thermal interface material which conducts heat by using carbon nanotubes, and a manufacturing method thereof.  
         [0003]     2. Description of Related Art  
         [0004]     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.  
         [0005]     A conventional thermal interface material is made by diff-using 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 modern electronic components.  
         [0006]     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.  
         [0007]     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 6600W/mK (watts/milliKelvin) at room temperature.  
         [0008]     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.  
         [0009]     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.  
         [0010]     A new thermal interface material which overcomes the above-mentioned problems and a method for manufacturing such material are desired.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     Accordingly, an object of the present invention is to provide a thermal interface material having a reduced thickness, good flexibility and excellent heat conduction.  
         [0012]     Another object of the present invention is to provide a method for manufacturing the above-described thermal interface material.  
         [0013]     To achieve the first of the above-mentioned objects, the present invention provides a thermal interface material comprising a silver colloid base and an array of carbon nanotubes embedded in the silver colloid base uniformly. The silver colloid base comprises silver particles, boron nitride particles and polysynthetic oils. The carbon nanotubes have nanometer-scale silver filled therein, are substantially parallel to each other, and extend from a first surface to a second surface of the thermal interface material.  
         [0014]     To achieve the second of the above-mentioned objects, a method for manufacturing the thermal interface material comprises the steps of:  
         [0015]     (a) forming an array of carbon nanotubes on a substrate, the carbon nanotubes having nanometer-scale silver filled therein;  
         [0016]     (b) submerging the carbon nanotubes in a silver colloid base;  
         [0017]     (c) solidifying the silver colloid base; and  
         [0018]     (d) peeling the solidified silver colloid base with the carbon nanotubes secured therein off from the substrate to obtain the thermal interface material.  
         [0019]     Unlike in a conventional thermal interface material, the carbon nanotubes of the thermal interface material of the present invention are disposed in the silver colloid base 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 carbon nanotubes have nanometer-scale silver filled therein, and this further enhances the thermal conductivity and heat conduction stability.  
         [0020]     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  
       [0021]      FIG. 1  is a schematic, side cross-sectional view of an array of carbon nanotubes directionally formed on a substrate, the carbon nanotubes having nanometer-scale silver filled therein;  
         [0022]      FIG. 2  is similar to  FIG. 1 , but showing the carbon nanotubes submerged in a silver colloid base;  
         [0023]      FIG. 3  is similar to  FIG. 2 , but showing the silver colloid base solidified with the carbon nanotubes embedded therein, and the silver colloid base being peeled off from the substrate;  
         [0024]      FIG. 4  is similar to  FIG. 3 , but only showing the thermal interface material of the present invention, the thermal interface material comprising the solidified silver colloid base and the carbon nanotubes disposed therein; and  
         [0025]      FIG. 5  is similar to  FIG. 4 , but showing the thermal interface material sandwiched between an electronic device and a heat sink. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]     Referring to  FIG. 1 , an array of carbon nanotubes  22  having nanometer-scale silver  24  filled therein is shown. The carbon nanotubes  22  are substantially parallel to each other and directionally formed on a substrate  11 .  
         [0027]     In a preferred method of the present invention, the carbon nanotubes  22  with the nanometer-scale silver  24  filled therein 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 method, 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 a catalyst film (not shown) is uniformly deposited on the substrate  11  by 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 method, the catalyst film  12  is made of iron.  
         [0028]     Secondly, the catalyst film is oxidized to obtain catalyst particles. The substrate  11  with the catalyst particles disposed thereon is placed in a reaction furnace. A carbon source gas with nanometer-scale silver mixed therein is introduced into the reaction furnace at a temperature of 350-1000° C. to grow the array of carbon nanotubes  22  with the nanometer-scale silver  24  filled therein. 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. The height of the array of carbon nanotubes  22  is generally in the range from 1 to 100 micrometers. In the preferred method of the present invention, the height of the array of carbon nanotubes  22  is about 100 micrometers. In the preferred method, the nanometer-scale silver  24  is nanometer-scale silver particles, and a purity thereof is about 99.9 percent. The nanometer-scale silver  24  is columnar, corresponding to the shapes of the carbon nanotubes  22 .  
         [0029]     Further 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.  
         [0030]     In an alternative method, the carbon nanotubes  22  with the nanometer-scale silver  24  filled therein are manufactured by an arc discharge method. Firstly, a catalyst film (not shown) is uniformly deposited on the substrate  11 , with a shape of the catalyst film being circular. Graphite poles with silver filled therein are provided as cathode electrodes and anode electrodes. Secondly, voltages are applied to the cathode electrodes and anode electrodes, thereby generating arc discharging. The anode electrodes are consumed, and the carbon nanotubes  22  with the nanometer-scale silver  24  filled therein are formed on the substrate  11 .  
         [0031]     In a further alternative method, the carbon nanotubes  22  with the nanometer-scale silver  24  filled therein are manufactured by a deposition method. Firstly, the array of carbon nanotubes  22  are directionally formed on the substrate  1 I by thermal chemical vapor deposition or plasma enhanced chemical vapor deposition. Secondly, an opening is formed in a free end of each carbon nanotube  22  by a physical method or a chemical method, as known in the art. Thirdly, the nanometer-scale silver  24  is filled in the carbon nanotubes  22  via the openings, thereby providing the carbon nanotubes  22  with the nanometer-scale silver  24  filled therein formed on the substrate  11 .  
         [0032]     In the following description, the formed carbon nanotubes  22  with the nanometer-scale silver  24  filled therein will simply be referred to as “the carbon nanotubes  22 .” 
         [0033]      FIG. 2  shows the carbon nanotubes  22  substantially submerged in a silver colloid base  32 . The silver colloid base  32  comprises nanometer-scale silver particles, nanometer-scale boron nitride particles and polysynthetic oils, and has a high heat conduction coefficient and a low volatility. Diameters of the nanometer-scale silver particles are in the range from 1 to 900 nanometers. A purity of the nanometer-scale silver particles is about 99.9%. Diameters of the nanometer-scale boron nitride particles are in the range from 1 to 900 nanometers. The nanometer-scale boron nitride particles can ensure stable heat conduction. A viscosity of the silver colloid base  32  is required to be below 100 cps (centipoise).  
         [0034]     Referring to  FIG. 3 , the silver colloid base  32  is cooled and solidified, and the solidified silver colloid base  32  with the carbon nanotubes  22  secured therein is peeled off from the substrate  11  to obtain the thermal interface material  40 . A thickness of the thermal interface material  40  is preferably about 100 micrometers, being equal to the height of the carbon nanotubes  22 . That is, the thickness of the thermal interface material  40  is determined by the height of the carbon nanotubes  22 . Thus, the thickness of the thermal interface material  40  can be varied by controlling the height of the carbon nanotubes  22 .  
         [0035]      FIG. 4  shows the thermal interface material  40  of the present invention in isolation. The thermal interface material  40  comprises the silver colloid base  32 , and the array of carbon nanotubes  22  embedded in the silver colloid base  32  uniformly. The thermal interface material  40  has a first surface  42  to contact a first thermal source, and a second surface  44  opposite to the first surface  42  to contact a second thermal source. The carbon nanotubes  22  are substantially parallel to each other, and extend from the first surface  42  to the second surface  44 . In the preferred embodiment, 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. Furthermore, because the carbon nanotubes  22  have the nanometer-scale silver  24  filled therein, the thermal conductivity and heat conduction stability of the thermal interface material  40  is further enhanced.  
         [0036]      FIG. 5  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 heat sink  60 , and the second surface  44  of the thermal interface material  40  engages with a surface (not labeled) of the electronic device  80 . 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 can provide good heat contact between the heat sink  60  and the electrical device  80 .  
         [0037]     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.