Abstract:
Methods for producing carbon nanotube thermal pads comprise forming an array of carbon nanotubes on a catalyst layer on a substrate and releasing the array from the substrate. The carbon nanotubes are grown so that they are generally vertically aligned relative to the substrate. Releasing the array can include dissolving the substrate. Alternately, a release layer between the substrate and the catalyst layer can be employed. The release layer can be chemically removed, or can provide a low-strength interface that is easily pulled apart or sheared.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/680,262 filed on May 11, 2005 and entitled “Carbon Nanotube-Based Thermal Pad,” U.S. Provisional Patent Application No. 60/691,673 filed on Jun. 17, 2005 and entitled “Carbon Nanotube-Based Thermal Pad,” and U.S. Provisional Patent Application No. 60/709,611 filed on Aug. 19, 2005 and entitled “Carbon Nanotube Based Interface Materials for Heat Dissipation Applications,” each of which is incorporated herein by reference in its entirety. This application is related to U.S. non-provisional patent application Ser. No. 11/______ filed on even date herewith and entitled “Methods for Forming Carbon Nanotube Thermal Pads” (attorney docket number PA3283US). This application is also related to U.S. non-provisional patent application Ser. No. 11/______ filed on even date herewith and entitled “Carbon Nanotube Thermal Pads” (attorney docket number PA3396US). This application is further related to U.S. non-provisional patent application Ser. No. 11/______ filed on even date herewith and entitled “Devices Incorporating Carbon Nanotube Thermal Pads” (attorney docket number PA3728US).  
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     This invention was made with United States Government support under Cooperative Agreement No. 70NANB2H3030 awarded by the Department of Commerce&#39;s National Institute of Standards and Technology. The United States has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention relates generally to the field of semiconductor packaging and more particularly to methods for forming structures that employ carbon nanotubes for thermal dissipation.  
         [0005]     2. Description of the Prior Art  
         [0006]     A carbon nanotube is a molecule composed of carbon atoms arranged in the shape of a cylinder. Carbon nanotubes are very narrow, on the order of nanometers in diameter, but can be produced with lengths on the order of hundreds of microns. The unique structural, mechanical, and electrical properties of carbon nanotubes make them potentially useful in electrical, mechanical, and electromechanical devices. In particular, carbon nanotubes possess both high electrical and thermal conductivities in the direction of the longitudinal axis of the cylinder. For example, thermal conductivities of individual carbon nanotubes of 3000 W/m-° K and higher at room temperature have been reported.  
         [0007]     The high thermal conductivity of carbon nanotubes makes them very attractive materials for use in applications involving heat dissipation. For example, in the semiconductor industry, devices that consume large amounts of power typically produce large amounts of heat. Following Moore&#39;s Law, chip integration combined with die size reduction results in an ever increasing need for managing power density. The heat must be efficiently dissipated to prevent these devices from overheating and failing. Presently, such devices are coupled to large heat sinks, often through the use of a heat spreader. Additionally, to allow for differences in coefficients of thermal expansion between the various components and to compensate for surface irregularities, thermal interface materials such as thermal greases are used between the heat spreader and both the device and the heat sink. However, thermal greases are both messy and require additional packaging, such as spring clips or mounting hardware, to keep the assembly together, and thermal greases have relatively low thermal conductivities.  
         [0008]     Therefore, what is needed are better methods for attaching heat sinks, sources, and spreaders that provides both mechanical integrity and improved thermal conductivity.  
       SUMMARY  
       [0009]     An exemplary method of forming a thermal pad comprises providing a substrate having a planar surface, forming a release layer on the planar surface, forming a catalyst layer on the release layer, forming an array of vertically aligned carbon nanotubes on the catalyst layer, and releasing the array of carbon nanotubes from the substrate. Releasing the array can include, for example, lifting the array off of the substrate with an adhesive layer, applying a shear force across the separation layer, or dissolving the release layer. Where releasing the array includes applying a shear force, the shear force can be applied mechanically or thermally by changing the temperature. Here, a difference in the coefficients of thermal expansion between the release layer and either the substrate, the catalyst layer, or some other layer between the two, creates the necessary shear force with the change of temperature to delaminate the array from the substrate. Advantageously, when the array is released, the newly freed surface is essentially as smooth as the planar surface of the substrate. The method can also comprise forming a surface layer, such as of copper, on the release layer before forming the catalyst layer. After release, the surface layer becomes one side of the thermal pad.  
         [0010]     Another exemplary method of forming a thermal pad comprises providing a substrate having a planar surface, forming a surface layer on the planar surface, forming a catalyst layer on the surface layer, forming an array of vertically aligned carbon nanotubes on the catalyst layer, and dissolving the substrate. The surface layer, in some embodiments, can include copper, zinc, aluminum, nickel, or silicon carbide, depending on the intended use of the thermal pad. For example, the surface composition can be selected to match a composition of a surface to which the thermal pad is intended to be mated. Dissolving the substrate can include etching the substrate with an acid such as hydrofluoric acid. In some embodiments, the substrate is segmented into coupons before the substrate is dissolved, and in some of these embodiments the coupon is attached to a heat management aid, such as a heat spreader or heat sink, or to a semiconductor die before dissolving the substrate.  
         [0011]     Still another exemplary method of forming a thermal pad comprises providing a substrate having a planar surface, forming a catalyst layer on the planar surface, forming an array of vertically aligned carbon nanotubes on the catalyst layer, and detaching the first end of the array from the catalyst layer. Detaching the first end of the array from the catalyst layer can include etching the carbon nanotubes at the first end of the array. Etching the carbon nanotubes can include subjecting the array to a heated atmosphere including hydrogen gas and water vapor.  
         [0012]     Each of the various exemplary methods can further comprise additional steps relating to forming the catalyst layer, such as forming a barrier layer and/or an interface layer before forming the catalyst layer. Likewise, each exemplary method can include steps such as infiltrating the array with a matrix material, patterning the catalyst layer, metallizing the exposed end of the array, or attaching the exposed end to a foil. These and other variations are disclosed in the following detailed description.  
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]      FIG. 1  shows a cross-sectional view of a partially completed thermal pad on a substrate according to an exemplary embodiment of the invention. The order of the layers, from bottom to top, serves to illustrate exemplary methods of forming the thermal pad.  
         [0014]      FIG. 2  shows a cross-sectional view of the thermal pad of  FIG. 1  after separation from the substrate.  
         [0015]      FIG. 3  shows a cross-sectional view of a partially completed thermal pad on a substrate according to another exemplary embodiment of the invention. The order of the layers, from bottom to top, serves to illustrate further exemplary methods of forming the thermal pad.  
         [0016]      FIG. 4  shows a cross-sectional view of the thermal pad of  FIG. 3  after separation from the substrate.  
         [0017]      FIG. 5  shows a top view of a partially completed thermal pad formed on a semiconductor wafer and the same wafer after dicing into coupons, according to an exemplary embodiment of the invention.  
         [0018]      FIG. 6  shows a cross-sectional view of a partially completed thermal pad on a substrate according to still another exemplary embodiment of the invention. The order of the layers, from bottom to top, serves to illustrate further exemplary methods of forming the thermal pad.  
         [0019]      FIG. 7  shows a cross-sectional view of the thermal pad of  FIG. 6  after separation from the substrate.  
         [0020]      FIGS. 8-15  show cross-sectional views of additional processing steps that can be performed on the partially completed thermal pads of  FIGS. 1, 4 , and  7  prior to release, according to various embodiments of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The present invention provides methods for fabricating free-standing carbon nanotube-based thermal pads. The various methods described herein provide free-standing thermal pads by forming the pads on a substrate from which the thermal pads can be later released. The thermal pads are characterized by an array of generally aligned carbon nanotubes where the direction of alignment is essentially perpendicular to the surfaces of array. The alignment of the nanotubes allows the array to provide excellent thermal conduction in the direction of alignment. Accordingly, a thermal pad between a heat source and a heat sink provides a thermally conductive interface therebetween.  
         [0022]     Some thermal pads are characterized by at least one, and in some instances, two very smooth surfaces. A thermal pad with a sufficiently smooth surface can adhere to another very smooth surface, such as the backside surface of semiconductor die, much like two microscope slides will adhere to each other. Surfaces of thermal pads, whether very smooth or not, can also be attached to an opposing surface with a metal layer, for example with solder, indium, or silver. Advantageously, some thermal pads are also characterized by a degree of flexibility and pliability. This can make it easier to work with the thermal pads in assembly operations and allows the thermal pads to conform to opposing surfaces that are curved or irregular.  
         [0023]      FIGS. 1 and 2  illustrate an exemplary method of forming a free-standing thermal pad. As shown by  FIG. 1 , a substrate  110  with a generally planar surface  120  is initially provided. Next, a separation layer  130  is formed on the planar surface  120 . The method can also include forming an optional barrier layer  140  either before or after forming the separation layer  130 . An optional interface layer  150  is formed over the separation layer  130 , and over the barrier layer  140 , if present.  
         [0024]     Next, a catalyst layer  160  is formed. The catalyst layer  160  can be formed either directly on the separation layer  130 , on the barrier layer  140 , or on the interface layer  150 . After the catalyst layer  160  has been formed, an array  170  of carbon nanotubes is formed on the catalyst layer  160 . The array  170  is formed such that the carbon nanotubes are generally aligned in a direction  175  perpendicular to the planar surface  120 . The array  170  includes a first end  180  attached to the catalyst layer  170  and a second end  190  opposite the first end  170 .  
         [0025]     Examples of a suitable substrate  110  include polished silicon and gallium arsenide wafers. Either can provide an atomically smooth planar surface  120  on which to form the successive layers  130 - 170 . An example of a suitable separation layer  130  is nickel oxide. A nickel oxide separation layer  130  can be formed by depositing and then passivating a nickel thin film to form a dense and continuous oxide film. The passivation can be achieved, for instance, by thermal oxidation, exposure to an oxygen plasma, or by exposure to a strong acid such as chromic acid. A suitable thickness for the nickel oxide separation layer  130  is about 100 Å.  
         [0026]     Another suitable separation layer  130 , where the substrate  110  is gallium arsenide, is aluminum arsenide. An aluminum arsenide separation layer  130  can be formed, for example, by metal oxide CVD (MOCVD), and a suitable thickness for such a film is about 500 Å. As an alternative to forming the separation layer  130  by deposition, the separation layer  130  can be formed by ion implantation into the substrate  110 . For example, hydrogen ions can be implanted into silicon to form a silicon hydride layer that can readily delaminate from the silicon.  
         [0027]     The purpose of the barrier layer  140  is to prevent diffusion between the substrate  110  and/or the separation layer  130  and the catalyst layer  160 . Preventing such diffusion is desirable in those embodiments where either the substrate  110  or the separation layer  130  includes one or more elements that can poison the catalyst of the catalyst layer  160  and prevent nanotube growth. Examples of elements that are known to poison nanotube catalysis include nickel, iron, cobalt, molybdenum, and tungsten. Other materials, such as silicon, are not known to poison nanotube catalysis. An example of a suitable barrier layer  140  is a sputtered film of aluminum oxide with a thickness of at least 50 Å, and more preferably 100 Å. An appropriate thickness for the barrier layer  140  will depend both on the permeability of the selected material to the elements to be impeded and on the roughness of the planar surface  120 , as rougher finishes require a thicker barrier layers  140 .  
         [0028]     The interface layer  150  is provided, where needed, to improve the catalyst layer  160  which, in turn, provides for higher quality nanotubes characterized by higher wall crystallinities and fewer defects. In some embodiments, a single layer can serve as both the barrier layer  140  and the interface layer  150 . Again, a sputtered film of aluminum oxide with a thickness of at least 50 Å, and more preferably 100 Å, can be a suitable interface layer  150 . Another suitable interface layer  150  includes silicon dioxide. It should be noted that too thick of an interface layer  150  can lead to cracking during thermal cycling due to mismatches in coefficients of thermal expansion between the interface layer  150  and the layer beneath.  
         [0029]     In various embodiments, the catalyst layer  160  is formed either directly on the separation layer  130 , on the barrier layer  140 , or on the interface layer  150 . After the catalyst layer  160  has been formed, an array  170  of carbon nanotubes is formed on the catalyst layer  160 . The array  170  is formed such that the carbon nanotubes are generally aligned in a direction  175  perpendicular to the planar surface  120 . The array  170  includes a first end  180  attached to the catalyst layer  160  and a second end  190  opposite the first end  180 . Depending on the growth conditions and choice of catalyst, the carbon nanotubes can be single-walled or multi-walled. The density, diameter, length, and crystallinity of the carbon nanotubes can also be varied to suit various applications.  
         [0030]     One general method for achieving carbon nanotube growth is to heat the catalyst layer  160  in the presence of a carbon-bearing gas. Examples of suitable catalysts and process conditions are taught, for example, by Erik T. Thostenson et al. in “Advances in the Science and Technology of Carbon Nanotubes and their Composites: a Review,” Composites Science and Technology 61 (2001) 1899-1912, and by Hongjie Dai in “Carbon Nanotubes: Opportunities and Challenges,” Surface Science 500 (2002) 218-214. It will be appreciated, however, that the present invention does not require preparing the carbon nanotubes by the catalysis methods of either of these references, and any method that can produce generally aligned carbon nanotubes extending from a surface is acceptable.  
         [0031]     After the carbon nanotube array  170  has been formed, the thermal pad is removed from the substrate  110  at the separation layer  130 , as shown in  FIG. 2 . It will be appreciated that the surface of the thermal pad, after release from the substrate  110 , will be essentially as smooth as the surface of the substrate  110  on which the array  170  was formed, and therefore can be atomically smooth, or nearly so. As described below, further processing of the array  170  can occur either before or after separation from the substrate  110 .  
         [0032]     Separation can be accomplished mechanically, chemically, or through a combination of techniques. For instance, where the separation layer comprises nickel oxide and an adjoining layer includes copper, separation can occur by the application of a shear force. Some embodiments take advantage of differences in the coefficients of thermal expansion between the array  170  and the separation layer  130 . In these embodiments, after the array  170  is formed and begins to cool, the mismatch in the coefficients of thermal expansion causes a stress to develop along the interface that can cause the array  170  to spontaneously detach, or to detach upon the application of very little force.  
         [0033]     An adhesive tape can be applied to the second end  190 , in some embodiments, to both pull the array  170  away from the separation layer  130  and to give the released thermal pad a backing layer. In some embodiments the adhesive tape becomes part of the completed thermal pad. In these thermal pads, suitable adhesive tapes can be either electrically insulating, such as with a Kapton backing, or metallized to be conductive, to allow the thermal pad to either electrically isolate a device or component, or provide a path to ground.  
         [0034]     Alternatively, the separation layer  130  can be removed chemically by wet etching or a thermal treatment. Thermal treatments take advantage of differences in coefficients of thermal expansion between layers to cause delamination at the separation layer  130 . Wet etching can be achieved with strong acids such as hydrofluoric acid to dissolve either silicon or silicon dioxide. For some applications less aggressive and more environmentally acceptable solvents are desirable. Water, for instance, can be used where the separation layer  130  comprises a water-soluble salt.  
         [0035]     As shown in  FIG. 3 , another exemplary method includes a surface layer  300  intended to become part of the finished thermal pad after separation from the substrate  110 . In  FIG. 3 , the surface layer  300  can be a thin layer of copper, for example. The particular material for the surface layer  300  can be chosen with respect to the intended use of the thermal pad. For instance, a metal for the surface layer  300  can be chosen to provide a superior bond to the material to which the thermal pad will be joined, or to match the coefficient of thermal expansion to that material. Zinc and silicon carbide are two examples of materials for the surface layer  300  where the thermal pad is to be attached to a silicon surface. Nickel and aluminum can be used for the surface layer  300  where the thermal pad will be joined to a nickel or aluminum coating on the surface of a thermal management aid (i.e., a heat sink or heat spreader).  
         [0036]     In the example of  FIG. 3 , compared to that in  FIGS. 1 and 2 , the surface layer  300  is substituted for the separation layer  130  and the thermal pad is separated from the substrate  110  by dissolving the substrate  110 , as shown in  FIG. 4 . For example, where the substrate  110  is a thinned silicon wafer, an acid such as hydrofluoric acid (HF) can be used to etch away the silicon, leaving the free-standing thermal pad bounded on one side by a smooth copper film. It will be appreciated that the surface layer  300  can also be added into the embodiment illustrated by  FIG. 1 . For instance, the surface layer  300  can be included between the separation layer  130  and the catalyst layer  160 . In some of these embodiments, the surface layer  300  can also serve as a barrier layer  140 .  
         [0037]     Although the method of  FIGS. 3 and 4  can produce a free-standing thermal pad, in some embodiments the free-standing thermal pad is never realized. Instead, the second end  190  is attached to a surface of another object such as a foil or a thermal management aid prior to the dissolution of the substrate  110 . Methods for attaching the second end  190  to other objects are discussed further below.  
         [0038]     In some of the embodiments represented generally by  FIGS. 1-4 , the substrate  110  is diced into sections or coupons before the thermal pad is separated from the substrate  110 .  FIG. 5  illustrates a top view of a wafer  500 , including an array  170 , and coupons  510  produced by dicing the wafer  500 . With respect to the embodiments of  FIGS. 1 and 2 , dicing can occur prior to separating the thermal pad from the substrate  110 . With respect to the embodiments of  FIGS. 3 and 4 , dicing can occur before the substrate  110  is dissolved. It will be understood that “dicing” is a term of art in the semiconductor field that is specific to cutting semiconductor wafers; however, the concept illustrated here is more general. Accordingly, a substrate  110  other than a semiconductor wafer can also be segmented prior to separating the substrate  110  from the thermal pad or before dissolving the substrate  110 . Metal substrates  110  can be die cut, for example.  
         [0039]     Yet another method for forming free-standing carbon nanotube thermal pads is illustrated in  FIGS. 6 and 7 . In  FIG. 6  an array  170  is formed on a substrate  110  in a manner analogous to that shown by  FIGS. 1 and 3 , but without either the separation layer  130  or the surface layer  300 . After the array  170  has been grown to the desired height, the environment (e.g., within a furnace or reactor) is modified to stop the growth of the carbon nanotubes and to etch the carbon nanotubes at the first end  180  of the array  170 . More specifically, the process chemistry in the environment is changed by eliminating the gaseous carbon source to stop the growth process, and by introducing another gas that preferentially etches the carbon nanotubes at the interface with the catalyst layer  160 .  
         [0040]     An example of such a gas is a mixture of water vapor and hydrogen gas. In one embodiment, the array  170  is grown in a tube furnace. 50 standard cubic centimeters per minute (sccm) of argon gas is bubbled through a water bubbler to saturate the argon with water vapor. The argon saturated with water vapor is then mixed with 400 sccm of hydrogen gas and introduced into the tube furnace which is maintained at a temperature of 700° C. This atmosphere is maintained in the tube furnace for 5 minutes and causes the thermal pad to lift off of the substrate  110 , as shown in  FIG. 7 .  
         [0041]     The array  170  in any of the above embodiments can be further processed by the methods described below either prior to, or after, release from the substrate  110 . In the following examples the embodiment shown in  FIG. 6  is further modified, but it will be understood that the illustrated further processes can also be applied to the embodiments of  FIGS. 1 and 3 .  
         [0042]      FIGS. 8 and 9  illustrate exemplary further steps to the embodiment of  FIG. 6 . In  FIG. 8 a  metal layer  800  is formed on the second end  190  of the array  170  so that the carbon nanotubes extend partially into the metal layer  800 . A suitable metal for the metal layer  800  is copper. The metal layer  800  can be formed, for instance, by sputtering, evaporation, or electroplating. It should be noted that the metal layer  800  is not meant to infiltrate the entire array  170  but only to encapsulate the very ends of the carbon nanotubes and to extend a short distance above the second end  190 . An appropriate thickness for the metal layer  800  will depend on the density of carbon nanotubes in the array  170  and the variation in their heights, but a minimum thickness for the metal layer  800  is on the order of 200 Å.  
         [0043]     In some embodiments, forming the metal layer  800  includes applying a conformal coating to the ends of the carbon nanotubes with a wetting layer of a metal that promotes improved wetting of the metal layer  800  to the carbon nanotubes. Suitable wetting layer materials include palladium, chromium, titanium, vanadium, hafnium, niobium, tantalum, magnesium, tungsten, cobalt, zirconium, and various alloys of the listed metals. The wetting layer can be further coated by a thin protective layer, such as of gold, to prevent oxidation of the wetting layer. The wetting and protection layers may be achieved by evaporation, sputtering, or electroplating, for example. It should be noted that these conformal coatings merely conform to the ends of the carbon nanotubes and are not continuous films across the second end  190  of the array  170 . Wetting and protection layers are described in more detail in U.S. non-provisional patent application Ser. No. 11/107,599 filed on Apr. 14, 2005 and titled “Nanotube Surface Coatings for Improved Wettability,” incorporated herein by reference in its entirety.  
         [0044]     The metal layer  800  can be polished to increase the smoothness of the surface.  FIG. 9  shows the structure of  FIG. 8  after metal layer  800  has been polished. Metal layer  800  is thus thinner in  FIG. 9  than in  FIG. 8 . Polishing the metal layer  800  can comprise chemical mechanical polishing (CMP) which also serves to planarize the surface. Copper is a good choice for the metal layer  800 , in those embodiments that include CMP of the metal layer  800  in that CMP of copper has been refined in the semiconductor processing arts. In some embodiments, polishing the metal layer  800  continues until the second end  190  of the array  170  is exposed, while in other embodiments polishing is discontinued before that point is reached, as shown in  FIG. 9 .  
         [0045]     As shown in  FIG. 10 , instead of forming and polishing a metal layer  800 , in other embodiments a thermal pad with a smooth surface is obtained by attaching a polished thin metal layer or foil  1000  to the array  170 . A foil is distinguished from the thin metal layer in that the thin metal layer is self-supporting while the foil is not. Attaching the foil  1000  can include forming an attachment layer  1010  on the second end  190  of the array  170  so that the carbon nanotubes extend partially into the attachment layer  1010 . Ideally, the attachment layer  1010  is formed of a low melting point metal or eutectic alloy such an indium, tin, bismuth, or a solder such as tin-silver, tin-lead, lead-silver, gold-germanium, or tin-antimony. The attachment layer  1010  may be formed by evaporation, sputtering, electroplating, or melting a thin sheet of the desired material, for example. As above, in some instances a wetting layer with or without a further protective layer can be applied as a conformal coating on the ends of the carbon nanotubes prior to forming the attachment layer  1010 .  
         [0046]     Copper and silver foils are examples of suitable foils  1000 . The foil  1000  can be joined to the attachment layer  1010  by heating the foil  1000  while in contact with the attachment layer  1010  to briefly melt the attachment layer  1010  at the interface. In some embodiments, such as those in which the low melting point metal comprises indium, it can be advantageous to strip the native oxide layer from the attachment layer  1010  by cleaning the attachment layer  1010  with an acid such as hydrochloric acid prior to attaching the foil  1000 .  
         [0047]     Each of the thermal pads shown in  FIGS. 2, 4 , and  7  is characterized by an array  170  of generally aligned carbon nanotubes with empty interstitial space between the carbon nanotubes. The empty interstitial space can be advantageous, in certain situations, as it provides the thermal pads with greater flexibility. In other embodiments, described below with reference to  FIGS. 11 and 12 , some or all of the interstitial space is filled.  
         [0048]     For example, in  FIG. 11  the interstitial space is filled by a matrix material  1100 . Examples of matrix materials include metals and polymers. The interstitial space of the array  170  can be filled by a metal, for example, by electroplating. Injection molding can be used, for instance, to fill the interstitial space of the array  170  with a polymer such as parylene. Polymer injection molding into aligned nanotubes is taught by H. Huang, C. Liu, Y. Wu, and S. Fan in Adv. Mater. 2005, 17, 1652-1656. Both metal and polymers can be useful to provide additional structural support, while metals also provide some additional thermal conductivity.  
         [0049]      FIG. 12  shows the interstitial space of the array  170  partially filled with a base metal layer  1200  that surrounds the carbon nanotubes at the first end  180  of the array  170  but otherwise leaves the interstitial space empty. The base metal layer  1200  can be formed of a metal such as copper by electroplating with the catalyst layer  160  serving as an electrode. The base metal layer  1200 , like the matrix material  1100 , is advantageous for further securing the array  170  to the catalyst layer  160 . The base metal layer  1200  both provides this advantage while still leaving much of the interstitial space empty for greater flexibility of the thermal pad.  
         [0050]      FIG. 13  illustrates yet another variation on the method of forming a thermal pad. In this example, the catalyst layer  160  is patterned, prior to forming the array  170 , so that the carbon nanotubes of the array  170  grow in columns or bundles  1300 . The catalyst layer  160  can be patterned, for example, by conventional masking techniques known to the semiconductor processing arts. Patterning the catalyst layer  160  to produce the bundles  1300  can be useful for those thermal pads that do not have a top layer such as metal layer  800  or foil  1000 . When the second end  190  of the array  170  of such a thermal pad is joined to a surface, the taller bundles  1300 , because of the spaces between the bundles  1300 , are able to bend until the shorter bundles  1300  also contact the surface. In a similar manner, bundles  1300  can be beneficial to thermal pads even with a top layer to allow the top layer to deform to match the contour of a mating surface.  
         [0051]     It should be noted that a continuous catalyst layer  160 , as shown for example in  FIG. 1 , can be patterned to include a varying composition, thickness, or density of catalyst particles. Examples of such patterned catalyst layers are described in more detail in U.S. non-provisional patent application Ser. No. 11/124,005 filed on May 6, 2005 and titled “Growth of Carbon Nanotubes to Join Surfaces,” incorporated herein by reference in its entirety. Providing such patterning can be advantageous to vary aspects of the carbon nanotubes within the array  170  as a function of location. For example, where the thermal pad is intended to provide an interface with a backside of a semiconductor die with a known curvature, such as a convex shape, the heights of the carbon nanotubes can be varied from shorter at the center of the array  170  to longer at the edges. Likewise, a greater density of carbon nanotubes can be grown in areas of the array  170  in order to match the greater density to hot spots on the heat source.  
         [0052]      FIGS. 14 and 15  illustrate still another variation on the method of forming a thermal pad. In this example, spacers  1400  are placed over the planar surface  120  of the substrate  110  before the array  170  is formed. In some embodiments, the spacers  1400  are placed on the catalyst layer  160  as shown in  FIG. 14 . Subsequently, the array  170  is formed, as shown in  FIG. 15 . Preferably, the array  170  is grown until a height of the array  170  exceeds a height of the spacers  1400 . A thermal pad including spacers  1400  can be advantageous during assembly of the thermal pad within a device, package, or other structure. Not only can the spacers  1400  provide an appropriate spacing between two objects such as a heat source and a heat sink, but the spacers  1400  can also prevent damage to the carbon nanotubes of the array  170  by limiting the extent to which the carbon nanotubes can be deformed during handling and assembly. Suitable spacers are described in more detail in U.S. Non-Provisional patent application Ser. No. 11/124,005 noted above.  
         [0053]      FIGS. 2, 4 , and  7  as shown or as further modified by the processes of  FIGS. 8-15  also represent different embodiments of finished thermal pads. The methods described herein are suitable to produce thermal pads with surface areas ranging from about 1 mm×1 mm, or less, to over 6″×6″. Arrays  170  of nanotubes can have thicknesses ranging from a few microns to over 1 mm. In particular, the thickness of the arrays  170  can be between 0.1 mm and 2 mm. Some thermal pads are characterized by a second end  190  with exposed nanotubes. Other thermal pads are characterized by a capped second end  190  where the capping is achieved with either an attached thin substrate or foil  1000 , or a metal layer  800  that is either unfinished, polished, or polished and planarized. Additionally, any of these thermal pads can include carbon nanotubes grown in bundles  1300 , and any can include spacers  1400 .  
         [0054]     Any of these thermal pads can include a matrix material  1100  that fills the interstitial space between the ends  180 ,  190  of the array  170 . Similarly, any can include a base metal layer  1200  that only partially fills the interstitial space of the array  170  around the carbon nanotubes at the first end  180 . Also, the interstitial space of any of these thermal pads can be left empty. As noted above, keeping the interstitial space empty improves flexibility. It should also be noted that keeping the interstitial space empty also improves compliance of the thermal pad to differential thermal expansion between opposing surfaces of two objects. The flexibility and pliability of some thermal pads allows them to be attached to curved surfaces in addition to generally flat surfaces.  
         [0055]     Some thermal pads are fixedly attached to inflexible substrates, such as heat spreaders, where the second end  190  of the array  170  is meant to be attached to the surface of some other object. Other such thermal pads are free-standing components meant to be disposed between the opposing surfaces of a heat source and a heat sink.  
         [0056]     A thermal pad having a second end  190  with exposed nanotubes can be joined to a surface of an object with a low melting point metal or eutectic alloy or a solder. One advantage of this method of joining the thermal pad to the surface is that neither the surface nor the second end  190  needs to be particularly smooth. Irregularities in either are filled by the low melting point metal, eutectic alloy, or solder. Reworking can be easily accomplished by low temperature heating.  
         [0057]     A thermal pad having a second end  190  with exposed nanotubes can also be joined to a surface of an object simply by pressing the two together, known herein as “dry-pressing.” Dry pressing can be accomplished with or without the addition of pressure and heat. Modest elevated temperatures (e.g. 200-300° C.) and pressures (e.g., 10 to 100 psi) can be used. In some embodiments, sufficient heat is applied to soften or melt the surface of the object, for example, the copper surface of a heat sink, so that the ends of the carbon nanotubes push into the surface. In these embodiments it can be advantageous to perform the dry-pressing in a non-oxidizing environment such as an oxygen-free atmosphere. Dry-pressing can also comprise making the ends of the carbon nanotubes temporarily reactive. Here, plasma etching can be used, for example, to etch away amorphous carbon and/or any catalyst materials. Plasma etching can also create reactive dangling bonds on the exposed ends of the carbon nanotubes that can form bonds with the opposing surface. Dry pressing can also comprise anodic bonding, where a strong electric field pulls ions from the interface to create a strong bond.  
         [0058]     Either end of a thermal pad that comprises a thin substrate, a foil  1000 , an unfinished metal layer  800  ( FIG. 8 ), or a polished metal layer  800  ( FIG. 9 ) can be joined to a surface of another object in several ways. One method is to join the surface of the object with a metal having a melting point below the melting points of the object and the opposing surface of the thermal pad. For example, silver can be used to join a copper heat spreader with a palladium metal layer  800 . Lower melting point metals such as indium and solder can also be used. In some embodiments the low melting point metal is cleaned with an acid such as hydrochloric acid to remove the native oxide. In the case of a thin substrate  200  comprising silicon, the silicon surface can be metallized with titanium and then silver to bond well to the low melting point metal.  
         [0059]     In other instances, where both the surface of the object and the exposed surface of the thin substrate or foil are very smooth, the two can be held together by van der Waals attractions. In still other instances, both the surface of the object and the exposed surface of the thin substrate or foil are compositionally the same or very similar, for example where both comprise silicon. In this example, Si—Si bonds can spontaneously form between the two surfaces.  
         [0060]     In the foregoing specification, the invention is described with reference to specific embodiments thereof, but those skilled in the art will recognize that the invention is not limited thereto. Various features and aspects of the above-described invention may be used individually or jointly. Further, the invention can be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the specification. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive. It will be recognized that the terms “comprising,” “including,” and “having,” as used herein, are specifically intended to be read as open-ended terms of art.