Thermal interface material and method for manufacturing same

[0001] This application is related to two corresponding U.S. patent applications each entitled “THERMAL INTERFACE MATERIAL AND METHOD FOR MANUFACTURING SAME” with application Ser. No. 11/024,513, filed on Dec. 28, 2004, and “METHOD FOR MANUFACTURING CARBON NANOTUBES WITH UNIFORM LENGTH” with application Ser. No. 11/025,160, filed on Dec. 28, 2004, each having the same assignee as the instant application. The disclosures of the above-identified applications are incorporated herein by reference.

[0002] 1. Field of the Invention

[0003] The invention relates generally to thermal interface materials and manufacturing methods thereof; and more particularly to a thermal interface material which conducts heat by using carbon nanotubes, and a manufacturing method thereof.

[0004] 2. Prior Art

[0005] 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.

[0006] 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.

[0007] 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 thickness of this kind thermal interface material is limited to be greater than 40 micrometers, and the heat conduction coefficient of the thermal interface material is inversely proportional to a thickness thereof. 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.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] A new thermal interface material which overcomes the above-mentioned problems and a method for manufacturing such material are desired.

[0012] 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.

[0013] Another object of the present invention is to provide a method for manufacturing the above-described thermal interface material.

[0014] To achieve the first of the above-mentioned objects, 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 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. Furthermore, two opposite ends of each carbon nanotube extend out of corresponding surfaces of the thermal interface material respectively and form two bent ends.

[0015] To achieve the second of the above-mentioned objects, a method for manufacturing the thermal interface material comprises the steps of:

• [0016] (a) forming an array of carbon nanotubes on a substrate;
• [0017] (b) submerging the carbon nanotubes in a liquid paraffin;
• [0018] (c) solidifying the liquid paraffin;
• [0019] (d) cutting the paraffin material in a direction perpendicular to long axes of the carbon nanotubes, thereby each carbon nanotube is open at one end thereof; and
• [0020] (e) cutting the paraffin in the same direction according to a predetermined thickness thereby each carbon nanotube is open at the other end thereof.

[0021] 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. Furthermore, two opposite ends of each carbon nanotube extend out of corresponding surfaces of the thermal interface material respectively and form two bent ends. The two bent ends are parallel to the surface of the thermal interface material. This ensures the carbon nanotubes can contact an electronic device and a heat sink directly. The thermal contact area between the thermal interface material and the electronic device is enlarged, and the thermal contact area between the thermal interface material and the heat sink is also enlarged. 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.

[0022] 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:

[0029] 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.

[0030] 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.

[0031] 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 a melted macromolecular solution, and a monomer solution of polymer. In the preferred embodiment, the liquid macromolecular material 32 is liquid paraffin. A viscosity of the liquid macromolecular material 32 is required to be below 200 cps (centipoise).

[0032] Referring to FIGS. 4 and 5, the substrate 11 having 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. 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. Thus, a thermal interface material 40 is formed.

[0033] 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. 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. Thus, a thermal interface material 40 is formed.

[0034] In detail, 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 and forms a bending 24. 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 and forms another bending 24. The bending direction of the two bent ends 24 depends on the cutting direction of the cutter. 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. The bent ends 24 are parallel to respective first and second surfaces 42, 44 (see FIG. 5) of the thermal interface material 40.

[0035] 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 the first surface 42 at a bottom thereof, and the second surface 44 at a top thereof 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. Furthermore, each carbon nanotube 22 is open at two ends thereof, and extends from the first surface 42 to the second surface 44 of the thermal interface material 40. Two opposite ends of each carbon nanotube 22 extend out of corresponding surfaces of the thermal interface material 40 respectively and form the two bent ends 24. The two bent ends 24 are parallel to the surface of the thermal interface material 40. This ensures that the carbon nanotubes can contact an electronic device and a heat sink directly. Therefore, the thermal interface material 40 has a high heat conduction coefficient and can conduct heat uniformly.

[0036] 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.

[0037] 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 can provide good heat contact between the heat sink 60 and the electrical device 80. Furthermore, each carbon nanotube 22 is open at two ends thereof, and extends from the first surface 42 to the second surface 44 of the thermal interface material 40. Two opposite ends of each carbon nanotube 22 extend out of corresponding surfaces of the thermal interface material 40 respectively and form the two bent ends 24. The two bent ends 24 are parallel to the first and second surfaces 42, 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. The thermal contact area between the thermal interface material 40 and the electronic device 80 is increased, and the thermal contact area between the thermal interface material 40 and the heat sink 60 is increased. 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.

[0038] 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.