Patent Publication Number: US-2009224422-A1

Title: Methods of fabricating a composite carbon nanotube thermal interface device

Description:
RELATED APPLICATION 
     This application is related to application Ser. No. ______, entitled “Method of Fabricating a Composite Carbon Nanotube Thermal Interface Device”, filed on even date herewith. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the packaging of an integrated circuit die and, more particularly, to methods for manufacturing a composite carbon nanotube structure that may be used as a thermal interface device. 
     BACKGROUND OF THE INVENTION 
     Illustrated in  FIG. 1  is a conventional packaged integrated circuit device  100 . The integrated circuit (IC) device  100  may, for example, comprise a microprocessor, a network processor, or other processing device, and the IC device  100  may be constructed using flip-chip mounting and Controlled Collapse Chip Connection (or “C4”) assembly techniques. The IC device  100  includes a die  110  that is disposed on a substrate  120 , this substrate often referred to as the “package substrate.” A plurality of bond pads on the die  110  are electrically connected to a corresponding plurality of leads, or “lands”, on the substrate  120  by an array of connection elements  130  (e.g., solder balls, columns, etc.). Circuitry on the package substrate  120 , in turn, routes the die leads to locations on the substrate  120  where electrical connections can be established with a next-level component (e.g., a motherboard, a computer system, a circuit board, another IC device, etc.). For example, the substrate circuitry may route all signal lines to a pin-grid array  125 —or, alternatively, a ball-grid array—formed on a lower surface of the package substrate  120 . The pin-grid (or ball-grid) array then electrically couples the die to the next-level component, which includes a mating array of terminals (e.g., pin sockets, bond pads, etc.). 
     During operation of the IC device  100 , heat generated by the die  110  can damage the die if this heat is not transferred away from the die or otherwise dissipated. To remove heat from the die  110 , the die is ultimately coupled with a heat sink  170  via a number of thermally conductive components, including a first thermal interface  140 , a heat spreader  150 , and a second thermal interface  160 . The first thermal interface  140  is coupled with an upper surface of the die  110 , and this thermal interface conducts heat from the die and to the heat spreader  150 . Heat spreader  150  conducts heat laterally within itself to “spread” the heat laterally outwards from the die  110 , and the heat spreader  150  also conducts the heat to the second thermal interface  160 . The second thermal interface  160  conducts the heat to heat sink  170 , which transfers the heat to the ambient environment. Heat sink  170  may include a plurality of fins  172 , or other similar features providing increased surface area, to facilitate convection of heat to the surrounding air. The IC device  100  may also include a seal element  180  to seal the die  110  from the operating environment. 
     The efficient removal of heat from the die  110  depends on the performance of the first and second thermal interfaces  140 ,  160 , as well as the heat spreader  150 . As the power dissipation of processing devices increases with each design generation, the thermal performance of these devices becomes even more critical. To efficiently conduct heat away from the die  110  and toward the heat sink  170 , the first and second thermal interfaces  140 ,  160  should efficiently conduct heat in a transverse direction (see arrow  105 ). 
     At the first thermal interface, it is known to use a layer of thermal grease disposed between the die  110  and the heat spreader  150 . Thermal greases are, however, unsuitable for high power—and, hence, high heat—applications, as these materials lack sufficient thermal conductivity to efficiently remove a substantial heat load. It is also known to use a layer of a low melting point metal alloy (e.g., a solder) as the first thermal interface  140 . However, these low melting point alloys are difficult to apply in a thin, uniform layer on the die  110 , and these materials may also exhibit low reliability. Examples of materials used at the second thermal interface include thermally conductive epoxies and other thermally conductive polymer materials. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional elevation view of a conventional integrated circuit package. 
         FIG. 2  is a block diagram illustrating one embodiment of a method of fabricating a composite carbon nanotube structure. 
         FIGS. 3A-3G  are schematic diagrams illustrating an embodiment of the method for fabricating a composite carbon nanotube structure, as shown in  FIG. 2 . 
         FIGS. 4A-4B  are schematic diagrams illustrating further embodiments of the method for fabricating a composite carbon nanotube structure, as shown in  FIG. 2 . 
         FIG. 5  is a block diagram illustrating a second embodiment of a method of fabricating a composite carbon nanotube structure. 
         FIGS. 6A-6F  are schematic diagrams illustrating an embodiment of the method for fabricating a composite carbon nanotube structure, as shown in  FIG. 5 . 
         FIG. 7  is a schematic diagram of a computer system including an integrated circuit device having a composite carbon nanotube structure constructed according to the method of  FIG. 2  or the method of  FIG. 5 . 
         FIG. 8  is a perspective view of an example of a conventional carbon nanotube. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrated in  FIGS. 2 through 6F  are embodiments of methods for fabricating a composite carbon nanotube structure that may be used as a thermal interface device in an IC device (e.g., the IC device  100  of  FIG. 1 ). In one of the disclosed embodiments, a number of carbon nanotubes are formed in a porous metal oxide layer that has been deposited on a sacrificial substrate. In a second disclosed embodiment, a composite carbon nanotube structure is grown on a substrate using a plating process, wherein carbon nanotubes are dispersed in the plating bath. The disclosed embodiments are explained below in the context of manufacturing thermal interface devices for IC chips; however, it should be understood that the disclosed thermal interface devices and the methods for their production may find application in a wide variety of applications where a thermally conductive element is needed or where a composite carbon nanotubes structure is desired (e.g., field emission displays, data storage devices, as well as other electronic and photonic devices). 
     An example of a typical carbon nanotube  800  is shown in  FIG. 8 . The carbon nanotube (or “CNT”) is generally cylindrical in shape and may be single walled or multi-walled. The carbon nanotube  800  extends along a primary axis  805 , and the nanotube  800  has a height  810  and a diameter  820 . The height  810  may be up to 50 μm in length for a multi-walled carbon nanotube and up to 2 cm in length for a single walled carbon nanotube. For multi-walled carbon nanotubes, the diameter  820  may be up to 100 nm, and for single walled carbon nanotubes, the diameter  820  may be up to 30 nm. Carbon nanotubes are characterized by high mechanical strength, good chemical stability, and high thermal conductivity, especially in a direction along their primary axis  805 . 
     Illustrated in  FIG. 2  is an embodiment of a method  200  of fabricating a composite carbon nanotube structure comprising an array of carbon nanotubes disposed within a porous metal oxide matrix. Also, the method  200  of  FIG. 2  is further illustrated in  FIGS. 3A through 3G , as well as  FIGS. 4A-4B , and reference should be made to these figures along with  FIG. 2 , as called out in the text. 
     Referring now to block  210  in  FIG. 2 , a sacrificial layer is formed on a substrate. This is illustrated in  FIG. 3A , where a sacrificial layer  320  has been formed on a substrate  310 . The sacrificial layer  320  may comprise any suitable material that will allow for separation of the final composite structure from the substrate  310 , as will be described in greater detail below. Materials suitable for the sacrificial layer include, by way of example, Vanadium (V), Titanium (Ti), Tungsten (W), and alloys thereof. The sacrificial layer  320  may be deposited using any suitable deposition technique, including chemical vapor deposition (CVD), physical vapor deposition (PVD) techniques such as sputtering, as well as electroplating and electroless plating. 
     The substrate  310  may comprise any suitable material upon which a composite carbon nanotube structure can be constructed, such as, for example, a silicon or a ceramic material. As noted above, in one embodiment, the composite carbon nanotube structure to be fabricated on the substrate  310  will ultimately be separated from the substrate. However, in other embodiments, the composite carbon nanotube structure is formed directly on a component, such as an integrated circuit die, a semiconductor wafer, a heat spreader, or a heat sink. 
     As set forth at block  220 , a metal layer is deposited on the sacrificial layer. This is illustrated in  FIG. 3B , where a metal layer  330  has been formed on the sacrificial layer  320 . In one embodiment, the metal layer  330  comprises Aluminum (Al). However, the metal layer  320  may comprise other suitable metals, including Nickel (Ni) or Silicon (Si). The metal layer  330  may be formed using any suitable deposition technique, including CVD, electroplating, electroless plating, or sputtering. 
     Referring to block  230 , the metal layer is anodized to form a porous metal oxide layer. This is shown in  FIG. 3C , where the metal layer  330  has been anodized to form a porous metal oxide layer  340 , and this metal oxide layer  340  includes a number of pores  342 . In one embodiment, where the metal layer  330  comprises Aluminum, the metal oxide layer  340  comprises Aluminum Oxide (Al 2 O 3 ). However, it should be understood that the metal oxide layer  340  may comprise an oxide of other metals (e.g., Nickel Oxide, Silicon Oxide). Any suitable anodization process may be employed to anodize the metal layer  330 . In one embodiment, the metal layer  330  is anodized in the presence of an acid (e.g., phosphoric acid, succinic acid, sulfuric acid, or oxalic acid) under a positive voltage in a range of between 1 and 60 volts. For Aluminum, as well as other metals, porosity in a range of between approximately 30% and 70% (by volume) can be achieved. 
     In  FIG. 3C , for ease of illustration, the metal layer  330  is represented as being fully anodized to a porous metal oxide layer  340 . However, it should be understood that, in practice, only portions of the metal layer  330  may be anodized to form a metal oxide. This is illustrated in  FIG. 4A , where portions of the metal layer  330  have been anodized to form metal oxide layer  340  including pores  342 , whereas other portions of the metal layer  330  remain unanodized. As shown in  FIG. 4A , at least a portion of the metal layer surrounding each pore  342  has been anodized to form a metal oxide  340 , and this layer of metal oxide surrounding the pores may be referred to as the “barrier layer.” 
     With reference still to  FIG. 4A , it can be seen that the bottom ends  344  of the pores  340  (or at least some of the pores) do not extend to the sacrificial layer  320 . As will be described below, carbon nanotubes will be grown in the pores  342  of metal oxide layer  340  and, upon separation from the sacrificial layer  320  and substrate  310 , carbon nanotubes grown in the pores  342  would not extend through the metal oxide layer  340  (i.e., their ends will be covered by a thin layer  349  of the metal oxide barrier layer). This thin layer of metal oxide remaining on the carbon nanotubes may affect the thermal performance of the resulting composite carbon nanotube structure. Accordingly, as shown at block  240 , excess material may be removed from the pores  342  of the metal oxide layer  340 . This is illustrated in  FIGS. 3D and 4B , where the thin layer  349  of metal oxide has been removed from the lower ends of the pores  340 , and the lower ends  346  of the pores  340  (see  FIG. 4B ) now extend into the sacrificial layer  320  (or at least to the sacrificial layer). Any suitable etching or other material removal process may be employed to remove excess material from the pores. 
     Returning now to  FIG. 2 , and block  250  in particular, a catalyst is selectively deposited within the pores  342  of the metal oxide layer  340 . This is illustrated in  FIG. 4B , where catalyst  350  has been deposited within the pores  342 . Note that, as shown in  FIG. 4B , the catalyst  350  has been selectively deposited on the exposed portion of the sacrificial layer  320  at the bottom  346  of the pore  340 . The catalyst  350  comprises any material upon which growth of a carbon nanotube can be initiated—i.e., the catalyst provides nucleation sites. Suitable catalysts include Iron (Fe), Nickel (Ni), Cobalt (Co), Rhodium (Rh), Platinum (Pt), Yttrium (Yt), and their combinations. 
     The selective deposition of the catalyst  350  may be achieved using either an electroplating process or an electroless plating process. In an electroplating process, no plating occurs on the exposed metal oxide surfaces within the pores  342  because sufficient electric current will not pass through the dielectric metal oxide. In an electroless plating process, the metal oxide material is not a catalytic material for the plating process, and the catalyst  350  does not build up on exposed metal oxide surfaces. For an electroplating process, the sacrificial layer  320  is comprised of an electrically conductive material and, for an electroless plating process, the sacrificial layer  320  is comprised of a suitable catalytic material (for the catalyst  350 ). 
     Referring now to block  260 , carbon nanotubes are formed in the pores of the metal oxide layer. This is illustrated in  FIG. 3E , where carbon nanotubes  360  have been formed in the pores  342  of metal oxide layer  340 . The carbon nanotubes will be selectively (or at least preferentially) grown on the catalyst  350  within the pores  342  of metal oxide layer  340 , and the carbon nanotubes will align themselves with the pores. Any suitable process may be employed to form the carbon nanotubes  360 , including CVD and plasma enhanced CVD (PECVD). Any suitable technique may be used to introduce carbon into the deposition chamber, including introducing a carbon-containing precursor (e.g., methane, ethylene, or acetylene), laser vaporization of carbon, electrical discharge between carbon electrodes, or gas phase CVD using carbon and metal carbonyls. The metal oxide layer  340  (and substrate  310 ) may also be heated during deposition (e.g., to a temperature of approximately 800° C.). 
     In one embodiment, as shown in  FIG. 3E , the carbon nanotubes  360  may be grown to a height that extends above the upper surface of the metal oxide layer  340 . The height of the carbon nanotubes  360  and the extent to which they extend above the upper surface of metal oxide layer  340  is generally a function of the deposition time. Extending the carbon nanotubes  360  above the metal oxide layer  340  may improve the thermal conductivity of the resulting composite carbon nanotube structure by providing improved contact between the carbon nanotubes  360  and any component (e.g., a die, heat spreader, or heat sink) to which they are coupled. In an alternative embodiment, rather than growing the carbon nanotubes  360  to a height above the metal oxide layer  340 , an etching process is performed to remove some of the metal oxide material, thereby exposing the ends of the carbon nanotubes. 
     In the embodiments described above, carbon nanotubes  360  are grown within the pores  342  of a porous metal oxide layer  340 . Metal oxides, such as Aluminum Oxide and oxides of other metals, are desirable because they can provide a regular and controlled pore structure. However, it should be understood that the disclosed embodiments are not limited to growth of carbon nanotubes in metal oxide materials. In further embodiments, carbon nanotubes may be grown in other porous substances (e.g., a porous polymer material). 
     As set forth at block  270 , the metal oxide matrix with carbon nanotubes is separated from the substrate to form a free-standing composite carbon nanotube structure. This is shown in  FIG. 3F , where the metal oxide layer  340  including carbon nanotubes  360  has been separated from the substrate  310  (and sacrificial layer  320 ) to form a free-standing composite carbon nanotube structure  300 . In one embodiment, this separation is accomplished by dissolution of the sacrificial layer  320 . The sacrificial layer  320  may be dissolved in a solution containing an acid (e.g., phosphoric acid, succinic acid, or sulfuric acid). Alternatively, the sacrificial layer  320  may be dissolved in an acid-containing solution in the presence of an anodic potential. The thickness of such a free-standing composite CNT structure  300  may, in one embodiment, be in a range of approximately 2 μm to 20 μm. 
     In a further embodiment, as set forth at block  280  in  FIG. 2 , the free-standing composite carbon nanotube structure is attached to another component (e.g., a die, a heat spreader, a heat sink, etc.). This is illustrated in  FIG. 3G , which shows a packaged IC device  301 . The packaged IC device  301  includes a first thermal interface device  300   a  disposed between an integrated circuit die  110  and a heat spreader  150 . The IC package  301  may also include another thermal interface device  300   b  disposed between the heat spreader  150  and a heat sink  170 . Each of the thermal interface devices  300   a ,  300   b  comprises a free-standing composite CNT structure, as shown in  FIG. 3F . Any suitable technique may be used to attach the composite CNT structure  300   a  (or  300   b ) to the die  110  and heat spreader  150  (or heat spreader  150  and heat sink  170 ). In one embodiment, a low melting point metal alloy (e.g., solder) is used to couple the composite CNT structure  300   a  (or  300   b ) with each of the die  110  and heat spreader  150  (or heat spreader  150  and heat sink  170 ), and the composite CNT structure may be mechanically pressed between these components (under, for example, a pressure in a range up to approximately 10 Kg/cm 2 ) to insure sufficient thermal contact is achieved. 
     Illustrated in  FIG. 5  is a second embodiment of a method  500  of fabricating a composite carbon nanotube structure. Also, the method  500  of  FIG. 5  is further illustrated in  FIGS. 6A through 6F , and reference should be made to these figures along with  FIG. 5 , as called out in the text. 
     Referring now to block  510  in  FIG. 5 , carbon nanotubes are dispersed within a plating solution. This is illustrated in  FIG. 6A , where a plating bath  605  includes a plating solution  680  to which carbon nanotubes  690  have been added. In one embodiment, the plating solution  680  is adapted for electroplating, and in another embodiment, the plating solution  680  is adapted for electroless plating. The carbon nanotubes  690  may, in one embodiment, comprise up to approximately 20 percent by weight of the plating solution  680 . Also, the solution  680  may be agitated to promote uniform dispersion of the carbon nanotubes  690 . 
     Note that, in  FIG. 6A , a substrate  610  has been disposed within the plating bath  605 . In one embodiment, the substrate  610  comprises an integrated circuit die. In another embodiment, the substrate  610  comprises a semiconductor wafer upon which integrated circuitry has been formed (that is to be cut into a number of IC die). In a further embodiment, the substrate  610  comprises a heat spreader (e.g., the heat spreader  150  shown in  FIG. 1 ), and in yet another embodiment, the substrate comprises a heat sink (e.g., the heat sink  170  of  FIG. 1 ). In yet a further embodiment, the substrate  610  comprises a sacrificial substrate that is ultimately separated from the structure formed thereon, as will be explained in more detail below. 
     For electroplating, the plating solution  680  comprises metal ions (of the metal to be plated on substrate  610 ) and an electrolyte, such as sulfuric acid (H 2 SO 4 ) or a base such as KOH (potassium hydroxide) or TMAH (tetramethylammonium hydroxide). The metal to be plated may comprise, by way of example, Tin (Sn), Indium (In), Copper (Cu), Nickel (Ni), Cobalt (Co), Iron (Fe), Cadmium (Cd), Chromium (Cr), Ruthenium (Ru), Rhodium (Rh), Rhenium (Re), Antimony (Sb), Bismuth (Bi), Platinum (Pt), Gold (Au), Silver (Ag), Zinc (Zn), Palladium (Pd), Manganese (Mn), or alloys thereof. In another embodiment, the plating solution  680  further comprises a complexing agent to complex ions in the plating solution in order to change their solubility and oxidation/reduction potential. For example, for Cobalt metal ions, citric acid can be used as the complexing agent to make the Cobalt ions soluble in a basic (high pH) solution. In a further embodiment, the plating solution  680  also includes one or more additives to regulate the material properties of the plated metal (e.g., polyethylene glycol or di-sulfides to regulate grain size). 
     For electroless plating, the plating solution comprises metal ions (again, of the metal to be plated on substrate  610 ), one or more complexing agents, and one or more reducing agents. As set forth previously, the metal to be plated may comprise Tin, Indium, Copper, Nickel, Cobalt, Iron, Cadmium, Chromium, Ruthenium, Rhodium, Rhenium, Antimony, Bismuth, Platinum, Gold, Silver, Zinc, Palladium, Manganese, or alloys thereof. Also as noted above, a complexing agent comprises a substance to complex ions in the plating solution in order to change their solubility and oxidation/reduction potential (see example above). The reducing agent (or agents) comprises any substance that will supply electrons to the plating bath  680  during the plating process, including formaldehyde, hypophosphite, dimethyl amine borane, or hydrazine hydrate. In another embodiment, the plating solution  680  also includes a substance to adjust the pH of the plating solution. In a further embodiment, the plating solution also includes one or more additives to regulate the properties of the deposited metal, as described above. 
     Referring next to block  520 , a layer of metal is plated on the substrate, wherein this metal layer includes carbon nanotubes from the plating bath. This is illustrated in  FIG. 6B , where a metal layer  620  has been formed on the substrate  610 , and this metal layer  620  includes a number of carbon nanotubes  690 . Thus, a metal matrix  620  having carbon nanotubes  690  dispersed therein is formed on the substrate  610 . The carbon nanotubes  690  in metal layer  620  originate from the plating solution  680 , and they are deposited on the substrate  610  along with the metal layer  620  during the plating process. Note that, in  FIG. 6B  (and  FIG. 6C ), the carbon nanotubes  690  are not shown in the plating solution  680  (although present in this solution), which has been done simply for clarity and ease of illustration. 
     The metal layer  620  may be deposited on the substrate using an electroplating process or an electroless plating process. For electroplating, in one embodiment, a seed layer may first be deposited on the substrate  610  prior to deposition of the metal layer  620 . This is shown in  FIG. 6C , where a seed layer  622  has been formed on the substrate  610 . The seed layer  622  will typically comprise the same metal that is to be plated on the substrate  610  (although the seed layer may be a different metal), and this seed layer  622  may be deposited using any suitable process (e.g., CVD). For electroless plating, a layer of catalyst  624  (also shown in  FIG. 6C ) may, in one embodiment, be deposited on the substrate  610  prior to plating. The catalyst layer may comprise a noble metal—e.g., Gold (Au), Palladium (Pd), Platinum (Pt), Ruthenium (Ru), Rhodium (Rh), Silver (Ag), Osmium (Os), or Iridium (Ir)—or a transition metal—e.g., Nickel (Ni), Cobalt (Co), or Iron (Fe)—or their alloys, and this layer may be deposited using any suitable process (e.g., CVD). Also, for electroplating, the plating solution  680  is typically maintained at room temperature, whereas for electroless plating, the plating solution  680  in plating bath  605  may be heated. 
     In one alternative embodiment, as set forth at block  530  in  FIG. 5 , an electric field is applied across the substrate during formation of the metal layer. This is illustrated in  FIG. 6D , where an electric field (E)  650  is applied across the substrate  610 . Any suitable device may be employed to apply the electric field  650  across the substrate  610 . For example, the substrate  610  may be disposed between two plates, wherein a voltage is applied between the two plates to create an electric field (similar to a parallel plate capacitor). In the presence of an electric field, a carbon nanotube will align itself with the electric field—i.e., the primary axis  705  (see  FIG. 7 ) of the carbon nanotubes will align in the direction of the electric field  650  (see arrow  652 )—and this alignment will be maintained during the plating process. In one embodiment, an electric field having a strength of approximately 10,000 V/cm is applied to align the carbon nanotubes; however, it should be understood that an electric field of any suitable strength may be applied, so long as the field induces the desired degree of alignment. As noted above, carbon nanotubes are excellent thermal conductors along their primary axis, and alignment of the carbon nanotubes  690  in a direction parallel (or at least substantially parallel) with the electric filed  650  will produce a metal matrix with carbon nanotubes that has a high thermal conductivity in the direction of alignment (again, see arrow  652 ). 
     In another embodiment, where the substrate  610  comprises a sacrificial substrate, the metal matrix layer  620  with carbon nanotubes  690  is separated from the substrate, as denoted at block  540 . This is shown in  FIG. 6E , where the metal matrix layer  620  with carbon nanotubes  690  has been separated from the substrate  610  to form a free-standing composite carbon nanotube structure  600 . In one embodiment, the thickness of this free-standing composite CNT structure  600  may be in a range of 2 μm to 20 μm. 
     In a further embodiment, the free-standing composite carbon nanotube structure  600  is attached to another component (e.g., a die, a heat spreader, a heat sink, etc.). This is illustrated in  FIG. 6F , which shows a packaged IC device  601 . The packaged IC device  601  includes a first thermal interface device  600   a  disposed between an integrated circuit die  110  and a heat spreader  150 . The IC package  601  may also include another thermal interface device  600   b  disposed between the heat spreader  150  and a heat sink  170 . Each of the thermal interface devices  600   a ,  600   b  comprises a free-standing composite CNT structure, as shown in  FIG. 6E . Any suitable technique may be used to attach the composite CNT structure  600   a  (or  600   b ) to the die  110  and heat spreader  150  (or heat spreader  150  and heat sink  170 ). In one embodiment, a low melting point metal alloy (e.g., solder) is deposited on a surface (or surfaces) of the composite CNT structure—see block  560  in FIG.  5 —and this layer of low melting point alloy is used to couple the composite CNT structure  600   a  (or  600   b ) with the die  110  and/or heat spreader  150  (or heat spreader  150  and/or heat sink  170 ). In another embodiment, the plated metal  620  itself comprises a low melting point metal or alloy, and attachment to the die  110  and/or heat spreader  150  (or heat spreader  150  and/or heat sink  170 ) is accomplished by re-melting the metal matrix layer  620 . 
     An IC device having a thermal interface comprising a free-standing composite CNT structure—e.g., the packaged IC device  301  of  FIG. 3G  having thermal interface devices  300   a ,  300   b , or the packaged IC device  601  of  FIG. 6F  having thermal interface devices  600   a ,  600   b —may find application in any type of computing system or device. An embodiment of such a computer system is illustrated in  FIG. 7 . 
     Referring to  FIG. 7 , the computer system  700  includes a bus  705  to which various components are coupled. Bus  705  is intended to represent a collection of one or more buses—e.g., a system bus, a Peripheral Component Interface (PCI) bus, a Small Computer System Interface (SCSI) bus, etc.—that interconnect the components of computer system  700 . Representation of these buses as a single bus  705  is provided for ease of understanding, and it should be understood that the computer system  700  is not so limited. Those of ordinary skill in the art will appreciate that the computer system  700  may have any suitable bus architecture and may include any number and combination of buses. 
     Coupled with bus  705  is a processing device (or devices)  710 . The processing device  710  may comprise any suitable processing device or system, including a microprocessor, a network processor, an application specific integrated circuit (ASIC), or a field programmable gate array (FPGA), or similar device. In one embodiment, the processing device  710  comprises an IC device including a free-standing composite CNT structure (e.g., packaged IC device  301  having thermal interface devices  300   a ,  300   b , or packaged IC device  601  having thermal interface devices  600   a ,  600   b ). However, it should be understood that the disclosed thermal interface devices comprising a composite CNT structure may find use in other types of IC devices (e.g., memory devices). 
     Computer system  700  also includes system memory  720  coupled with bus  705 , the system memory  720  comprising, for example, any suitable type of random access memory (e.g., dynamic random access memory, or DRAM). During operation of computer system  700  an operating system  724 , as well as other programs  728 , may be resident in the system memory  720 . Computer system  700  may further include a read-only memory (ROM)  730  coupled with the bus  705 . During operation, the ROM  730  may store temporary instructions and variables for processing device  710 , and ROM  730  may also have resident thereon a system BIOS (Basic Input/Output System). The computer system  700  may also include a storage device  740  coupled with the bus  705 . The storage device  740  comprises any suitable non-volatile memory—such as, for example, a hard disk drive—and the operating system  724  and other programs  728  may be stored in the storage device  740 . Further, a device  750  for accessing removable storage media (e.g., a floppy disk drive or CD ROM drive) may be coupled with bus  705 . 
     The computer system  700  may include one or more input devices  760  coupled with the bus  705 . Common input devices  760  include keyboards, pointing devices such as a mouse, and scanners or other data entry devices. One or more output devices  770  may also be coupled with the bus  705 . Common output devices  770  include video monitors, printing devices, and audio output devices (e.g., a sound card and speakers). Computer system  700  further comprises a network interface  780  coupled with bus  705 . The network interface  780  comprises any suitable hardware, software, or combination of hardware and software capable of coupling the computer system  700  with a network (or networks)  790 . 
     It should be understood that the computer system  700  illustrated in  FIG. 7  is intended to represent an exemplary embodiment of such a computer system and, further, that this computer system may include many additional components, which have been omitted for clarity and ease of understanding. By way of example, the computer system  700  may include a DMA (direct memory access) controller, a chip set associated with the processing device  710 , additional memory (e.g., a cache memory), as well as additional signal lines and buses. Also, it should be understood that the computer system  700  may not include all of the components shown in  FIG. 7 . 
     Embodiments of a methods  200 ,  500  for fabricating composite carbon nanotube structures  300 ,  600 —as well as embodiments of a thermal interface device comprising such a composite CNT structure—having been herein described, those of ordinary skill in the art will appreciate the advantages of the disclosed embodiments. The disclosed composite CNT structures provides high thermal conductivity, high mechanical strength, and good chemical stability. Further, these composite CNT structures may be fabricated to a very thin and uniform thickness. Also, the disclosed composite CNT structures may be fabricated using well known, low cost methods (e.g., CVD, PECVD, electroplating, electroless plating, sputtering, etc.), and their fabrication and use as thermal interface devices is compatible with existing assembly and process conditions. 
     The foregoing detailed description and accompanying drawings are only illustrative and not restrictive. They have been provided primarily for a clear and comprehensive understanding of the disclosed embodiments and no unnecessary limitations are to be understood therefrom. Numerous additions, deletions, and modifications to the embodiments described herein, as well as alternative arrangements, may be devised by those skilled in the art without departing from the spirit of the disclosed embodiments and the scope of the appended claims.