Abstract:
A method for making a thermal interface structure which includes a carbon nanotube layer, in which the carbon nanotubes are oriented parallel to the direction of thermal transmission and metal layers provided on two edge surfaces of the carbon nanotube layer, the edge surfaces being perpendicular to the direction of the thermal transmission and located substantially parallel to the orientation direction at which edges of the carbon nanotubes are oriented.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a method for manufacturing a thermal conduction structure. Specifically, the present invention relates to fabricating a thermal interface structure capable of being used in a thermal conduction module in which integrated circuit (IC) chips or the like are embedded. 
       BACKGROUND OF THE INVENTION 
       [0002]    In recent years, the power consumption of semiconductor ICs has continued to increase with the development of higher-density ICs. The increase in the electric power leads to an increase in the amount of heat generated, and then results in one of the reasons to hinder the improvement in clock frequencies of the semiconductor ICs. For this reason, the semiconductor ICs need to be cooled at a high efficiency for further improvement in clock frequencies of the semiconductor ICs and the like. As a structure for cooling a semiconductor IC, a thermal contact material (thermal interface structure) is provided between the semiconductor IC and a heat radiating mechanism (heat sink) to mitigate the influence of thermal expansion. The thermal resistance at this interface is high, and makes up about a half of the thermal resistance in the entire cooling system. Accordingly, what has been longed for is a thermal interface structure with thermal resistance as low as possible. 
         [0003]    In such a circumstance, a carbon nanotube (hereinafter referred to as “CNT”), which has a high thermal conductivity and high mechanical flexibility, is expected to be used as the thermal contact material. H. Ammita et al., “Utilization of carbon fibers in thermal management of Microelectronics,” 2005 10th International Symposium on Advanced Packaging Materials: Processes, Properties and Interfaces, 259 (2005) discloses a use of CNTs as a thermal contact material (grease) by incorporating the CNTs into fats, oils, or the like. U.S. Pat. No. 6,965,513 discloses that CNTs orientationally grown are used as a thermal contact material into which the CNTs are formed by binding with an elastomer or the like. However, in any of these disclosures, a low thermal resistance value down to a practical level is not obtained. This is because there exists a high contact resistance between the CNTs and the substrate with which the CNTs come into contact. For this reason, a method is demanded in which a low thermal resistance (high thermal coupling) is achieved between CNTs and the substrate. 
       SUMMARY OF THE INVENTION 
       [0004]    An object of the present invention is to provide a thermal interface structure with a low thermal resistance. 
         [0005]    Another object of the present invention is to provide a thermal conduction module with a high thermal conduction efficiency. 
         [0006]    The present invention provides a thermal interface structure which includes: an oriented carbon nanotube layer; and metal layers respectively provided on two surfaces of the carbon nanotube layer, the surfaces being located in the directions to which edges of the carbon nanotubes are oriented (hereinafter, the surfaces will be referred to as “edge surfaces”). 
         [0007]    The present invention provides a thermal conduction module which includes: a heating body; a radiator; and a thermal interface structure provided between the heating body and the radiator. The thermal interface structure includes: a carbon nanotube layer in which the carbon nanotubes are oriented substantially parallel to a direction of thermal flow from the heating body to the radiator; a first metal layer connected to one of the lateral edge surfaces of the carbon nanotube layer, substantially perpendicular to the orientation of the carbon nanotubes, and thermally connected to the heating body; and a second metal layer connected to the other of the edge surfaces of the carbon nanotube layer substantially perpendicular to the orientation of the carbon nanotubes, and thermally connected to the radiator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    For a more complete understanding of the present invention and the advantage thereof, reference is now made to the following description taken in conjunction with the accompanying drawings. 
           [0009]      FIG. 1  is a diagram showing a cross section of a thermal interface structure of the present invention. 
           [0010]      FIG. 2  is a diagram showing a cross section of a thermal conduction module of the present invention. 
           [0011]      FIG. 3  is a diagram showing a method of manufacturing a thermal interface structure of an embodiment of the present invention. 
           [0012]      FIG. 4  is a diagram showing another method of manufacturing a thermal interface structure of an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    In the present invention, in order to reduce contact resistance, metal layers are provided between surfaces of a CNT layer and of a substrate or the like which faces the CNT layer. The metal layers are formed by, for example, a sputtering method as continuous metal layers on the surfaces of the layer of CNTs that are orientationally grown. Furthermore, the surfaces of the metal layers can further be thermally coupled to a substrate or the like by use of a low-melting-point metal, for example. With these components, the present invention accomplishes a thermal conduction structure with a low thermal resistance. The orientation, the high thermal conductivity and the mechanical flexibility of the CNTs are fully utilized to accomplish the above-mentioned goal. The present invention will be described in detail below with reference to the appended drawings. 
         [0014]      FIG. 1  shows a cross section of a thermal interface structure  10  of the present invention. The thermal interface structure  10  includes a CNT layer  1  and metal layers  2  and  3 . The CNTs of the CNT layer  1  are oriented substantially parallel to a direction of thermal transmission (i.e., the vertical direction as shown in  FIG. 1 ). The CNT is a one-dimensional thermal conductive substance. Although the thermal conductivity in a direction of the longitudinal axis of the tube of the CNT is considerably large, the thermal conductivity in a direction perpendicular to the longitudinal axis (that is, horizontal direction) is small. Thus, in the present invention, the direction in which the CNTs of the CNT layer are oriented is preferably a direction parallel to the direction of the longitudinal axis of the tube of the CNT and parallel to the desired direction of thermal transmission. The metal layers  2  and  3  are respectively joined to the upper surface and lower surface of the CNT layer  1 , substantially perpendicular to the orientation of the CNTs. The metal layers are preferably made of a metal selected from the group consisting of Au, Ni and Pt. Other metals, such as Ag, may be used as the metal layers. In order to increase the mechanical strength of the CNT layer, an elastic material such as a Si elastomer can be interspersed between the CNTs of the CNT layer  1 . 
         [0015]      FIG. 2  shows a cross section of a thermal conduction module  20  of the present invention.  FIG. 2  shows that the thermal interface structure  10  shown in  FIG. 1  is used. The metal layer  2  on the upper side of the thermal interface structure is connected to a heat sink  6  with a low-melting-point metal material (for example, Ga, an alloy thereof, or the like) or a solder material (for example, Pb—Sn) interposed therebetween. Herein, the low-melting-point metal material or the solder material is denoted by the reference numeral  4 . Likewise, the metal layer  3  on the lower side of the thermal interface structure is connected to a heating body  7  with a low-melting-point metal material or a solder material interposed therebetween. In this case, the low-melting-point metal material or the solder material is denoted by the reference numeral  5 . The heating body  7  is, for example, a semiconductor IC (IC chip). The heat sink  6  is made of a material with a high thermal conductivity such as aluminum. An example of the IC chip includes micro-processor unit (MPU) or the like. 
         [0016]      FIG. 3  shows an embodiment of a method of manufacturing the thermal interface structure of the present invention. In step (a), on a Si substrate  31 , CNTs of a CNT layer  32  are grown oriented in the vertical direction. The CNTs are grown, for example, in a container for the thermal CVD into which an acetylene gas is introduced while the substrate temperature is set at 750° C. The thickness of the CNT layer  32  is approximately 30 μm to 150 μm. In step (b), a metal layer  33  is formed on a surface of the CNT layer  32 . For example, by the use of a sputtering apparatus, an Au layer is formed in a thickness of approximately 1 μm. The thickness of the metal layer  33  may be approximately 0.5 μm to 5 μm. This relatively thick metal layer  33  improves the thermal coupling as well as the mechanical strength of the CNT layer  32 . Accordingly, a disturbance of the orientation of the CNTs is prevented. In step (c), a liquid metal layer  34  (for example, Ga) is coated on a surface of the metal layer  33 . In step (d), the substrate  31  is joined to a metal (for example, copper) block  35  so that the liquid metal layer  34  can come into contact with a surface of the metal block  35 . Thereafter, the entire structure or a portion thereof corresponding to the liquid metal layer  34  is cooled from the outside to solidify the liquid metal layer  34 . The cooling temperature is, for example, not higher than approximately 4° C. in a case of a Ga-based liquid metal. Due to this solidification, the substrate  31  (the CNT layer  32 ) and the metal block  35  are coupled to each other with the liquid metal layer  34  interposed therebetween. Note that, instead of cooling the entire structure or the portion thereof corresponding to the liquid metal layer  34  from the outside, the metal block  35  may be prepared in advance by cooling down to the temperature at which or below which the liquid metal layer  34  can be solidified. Subsequently, the liquid metal layer  34  is joined to the surface of the metal block  35 . 
         [0017]    In step (e), the substrate  31  and the CNT layer  32  are separated from each other by removing the substrate  31  from the CNT layer  32 . In step (f), the entire structure or the portion thereof corresponding to the liquid metal layer  34  is heated from the outside to melt the solidified liquid metal layer  34 . Then, the CNT layer  32  is separated from the metal block  35 . In step (g), the melted liquid metal layer  34  is removed from the surface of the metal layer  33 . In step (h), on the exposed surface of the CNT layer  32 , a metal layer  36  is formed in a similar way to that in the case of step (b). Through a series of the steps described above, a thermal interface structure using the CNT layer is manufactured. Note that, after step (g), a flowable elastic material such as a Si elastomer may be impregnated in each gap between the CNTs of the CNT layer  32  in a vacuum container. Due to the solidification of the elastic material, the mechanical strength of the CNT layer  32  can be increased. 
         [0018]      FIG. 4  shows another embodiment of the method of manufacturing the thermal interface structure of the present invention. Steps (a) and (b) are the same as in the case of  FIG. 3 . In step (c), on the surface of the metal layer  33 , an ultraviolet-removable (UV-removable) tape  40  is attached. The UV-removable tape is an adhesive tape with which an adhesion layer thereof can be removed from a target to be adhered. Specifically, the adhesion layer is degraded by irradiating with a UV light to generate a gas (e.g., an air bubble) by which the adhesion layer is removed therefrom. In step (d), the substrate  31  and the CNT layer  32  are separated from each other by removing the substrate  31  from the CNT layer  32 . In step (e), by irradiating the UV-removable tape  40  with a UV, the adhesion layer is degraded. In step (f), the UV-removable tape  40  and the metal layer  33  are separated from each other by removing the UV-removable tape  40  from the surface of the metal layer  33 . At this time, in a case where a residue of the adhesion agent remains on the surface of the metal layer  33  after the removal, the residue is removed by ozone cleaning or the like. In step (g), on the surface of the CNT layer  32 , the metal layer  36  is formed as in the case of step (h) shown in  FIG. 3 . Through a series of the steps described above, a thermal interface structure using the CNT layer is manufactured. Note that, after step (g), in a vacuum container, an elastic material such as a Si elastomer may be impregnated in each gap between the CNTs of the CNT layer  32 . Due to the solidification of the elastic material, the mechanical strength of the CNT layer  32  can be increased. 
         [0019]    A measurement was made on a thermal resistance of the thermal interface structure manufactured according to the method shown in  FIG. 3 . The steady state method was used in the measurement. The steady state method is one generally in which a constant joule heat is provided to a sample to obtain a thermal conductivity based on a heat flux Q and a temperature gradient ΔT at the time of providing the heat. The sample had an area of 10 mm×10 mm, and a thickness of several tens of micrometers to a hundred micrometers. The sample was sandwiched between two copper blocks having a thermocouple. One end of the copper blocks was heated with a heater, and the other end was cooled with the heat sink. Between both ends, a constant heat flux Q was generated to measure a temperature gradient ΔT at that time. A thermal resistance R was obtained according to the formula R=ΔT/Q. To be more specific, the values of ΔT corresponding to a plurality of Qs were plotted on a graph, and the thermal resistance R was obtained by linearly fitting (approximating) the values. The actually obtained thermal resistance value was 18 mm 2 K/W (film thickness: 80 μm). The thermal resistance values in a case of using CNT-coated Si as shown in  FIG. 8  of, or in a case of using CNT-coated Cu(Si) as shown in  FIG. 10  of, the above described document “Utilization of carbon fibers in thermal management of Microelectronics” were respectively 110 mm 2 K/W or 60 mm 2 K/W. Compared with the document, the thermal resistance value of the present invention was not larger than about one-third of these thermal resistance values. 
         [0020]    The present invention has been described with reference to the drawings. However, the present invention is not limited to these embodiments described above. It will be apparent to those skilled in the art that any modification can be made without departing from the spirit and scope of the present invention.