Patent Publication Number: US-2007116626-A1

Title: Methods for forming carbon nanotube thermal pads

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      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.  
      2. Description of the Prior Art  
      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.  
      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.  
      Therefore, what is needed are better methods for attaching heat sinks, sources, and spreaders that provides both mechanical integrity and improved thermal conductivity.  
     SUMMARY  
      An exemplary method of forming a thermal pad comprises providing a substrate having a thickness of less than 500μ and a planar surface, forming a catalyst layer over the planar surface of the substrate, and forming an array of carbon nanotubes on the catalyst layer. The array is formed such that the carbon nanotubes are generally aligned in a direction perpendicular to the planar surface. The array thus formed is characterized by a first end attached to the catalyst layer and a second end opposite the first end.  
      The substrate is preferably thin and in some embodiments is a copper foil or a thinned silicon wafer. The thickness of the substrate can be less than 500μ, less than 250μ, or less than 100μ. In some embodiments, an interface layer is formed on the substrate before the catalyst layer is formed. In some of these embodiments a barrier layer is formed on the substrate before the interface layer is formed. The catalyst layer can be patterned so that the array forms bundles of aligned carbon nanotubes on the patterned catalyst layer. Spacers can also be provided on the planar surface before forming the array so that the finished thermal pad will include spacers that can serve to protect the carbon nanotubes of the array from damage during handling and assembly.  
      Variations on the method include infiltrating a matrix material into the array to fill an interstitial space between the first and second ends. Alternately, a base metal layer can be formed around the carbon nanotubes at the first end of the array such that the interstitial space between the base metal layer and the second end of the array remains unfilled. In some embodiments the interstitial space advantageously remains unfilled. In some further embodiments a catalyst layer is formed on both sides of the substrate and then an array of carbon nanotubes is formed on each.  
      The carbon nanotubes at the second end of the array can be left free in the finished thermal pad. In some embodiments, however, a metal layer is formed on the second end of the array such that the carbon nanotubes extend at least partially into the metal layer. This metal layer can then be polished to make it smooth. Forming this metal layer can include coating the ends of the carbon nanotubes with a wetting layer. The wetting layer can, in turn, be coated with a protective layer over the wetting layer. Instead of a deposited metal layer, in some embodiments a metal foil is attached to the second end of the array. Attaching the metal foil can include, in some embodiments, forming an attachment layer on the second end of the array.  
      In some embodiments the substrate is a foil. The foil can be supported and handled according to several different embodiments. For example, the foil can be supporting with a frame. In other embodiments, the foil is fed from a roll into a guide and a transport mechanism is used to move the foil along the guide.  
      Another exemplary method of forming a thermal pad comprises providing a lead frame having a die bonding pad, forming a catalyst layer over the die bonding pad, and forming an array of carbon nanotubes on the catalyst layer such that the carbon nanotubes are generally aligned in a direction perpendicular to the die bonding pad. In some embodiments, forming the array comprises heating the die bonding pad by applying a current to the die bonding pad. Also in some embodiments the method further comprises separating the die bonding pad from the lead frame.  
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIGS. 1-11  show cross-sectional views of thermal pads according to various exemplary embodiments of the invention. The orders of the layers, from bottom to top, in each of these drawings also serve to illustrate exemplary methods of forming the thermal pads.  
       FIG. 12  shows a cross-sectional view of a partially completed thermal pad according to an exemplary embodiment of the invention.  
       FIG. 13  shows a cross-sectional view of the thermal pad of  FIG. 12  after an array of vertically aligned carbon nanotubes has been fabricated according to an exemplary embodiment of the invention.  
       FIG. 14  shows a cross-sectional view of still another thermal pad according to an exemplary embodiment of the invention.  
       FIG. 15  shows a top view of a portion of a lead frame used as a substrate for forming a thermal pad according to an exemplary embodiment of the invention.  
       FIG. 16  shows a cross-sectional view of a plurality of lead frames disposed in a tube furnace for carbon nanotube synthesis thereon, according to an exemplary embodiment of the invention.  
       FIG. 17  shows a cross-sectional view of the lead frames and furnace of  FIG. 16  taken along the line  17 - 17 .  
       FIG. 18  shows an enlarged view of a portion of the cross-sectional view of  FIG. 17 .  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention provides methods for fabricating carbon nanotube-based thermal pads. The thermal pads are characterized by an array of generally aligned carbon nanotubes disposed on a substrate, such as a foil, a thin metal sheet, or the surface of a component of a device. The carbon nanotubes are disposed on the substrate such that the direction of alignment is essentially perpendicular to the surface of the substrate on which the array is disposed. 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.  
      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.  
       FIG. 1  illustrates an exemplary method of forming a thermal pad. In the exemplary method a substrate  110  with a generally planar surface  120  is initially provided. Various examples of suitable substrates  110  are described below. Next, an optional barrier layer  125  is formed on the planar surface  120 . The purpose of the barrier layer  125  is to prevent diffusion between the substrate  110  and a subsequently deposited catalyst layer. Preventing such diffusion is desirable in those embodiments where the substrate  110  includes one or more elements that can poison the catalyst and prevent nanotube growth. Examples of elements that are known to poison nanotube catalysis include nickel, iron, cobalt, molybdenum, and tungsten. Other substrates, such as silicon, are not known to poison nanotube catalysis and may not therefore require the barrier layer  125 . An example of a suitable barrier layer  125  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  125  will depend both on the permeability of the selected material to the elements to be impeded, and also on the roughness of the planar surface  120 , as rougher finishes require thicker barrier layers  125 .  
      An optional interface layer  130  is formed over the planar surface  120 , and over the barrier layer  125 , if present. The interface layer  130  is provided, where needed, to improve the subsequent catalyst layer 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  125  and the interface layer  130 . 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  130 . Another suitable interface layer  130  includes silicon dioxide. It should be noted that too thick of an interface layer  130  can lead to cracking during thermal cycling due to mismatches in coefficients of thermal expansion between the interface layer  130  and the layer beneath.  
      Next, a catalyst layer  140  is formed. The catalyst layer  140  can be formed either directly on the planar surface  120  of the substrate  110 , on the barrier layer  125 , or on the interface layer  130 , depending on the various materials chosen for the substrate  110  and the catalyst layer  140 . After the catalyst layer  140  has been formed, an array  150  of carbon nanotubes is formed on the catalyst layer  140 . The array  150  is formed such that the carbon nanotubes are generally aligned in a direction  155  perpendicular to the planar surface  120 . The array  150  includes a first end  160  attached to the catalyst layer  140  and a second end  170  opposite the first end  160 . 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.  
      One general method for achieving carbon nanotube growth is to heat the catalyst layer  140  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 - 241. 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.  
       FIGS. 2-5  illustrate the method set forth with respect to  FIG. 1  as applied to specific substrates. In  FIG. 2 a  substrate  200  represents either a thin substrate or a foil. Both a foil and a thin substrate are characterized by the planar surface  120  and an opposing planar surface  210 . In some embodiments, the planar surface  210  has an optically smooth finish. The distinction between a foil and a thin substrate is that the thin substrate is self-supporting while the foil is not. Thus, a foil should be secured to a supporting structure such as a pedestal or a frame during processing, while a thin substrate need not be secured. Copper and silver foils are examples of suitable foils. Suitable thin substrates include polished metal blanks and semiconductor wafers. For example, a 4″ single-crystal silicon wafer can be thinned by conventional backside thinning processes, like grinding followed by chemical mechanical polishing (CMP), to a thickness of 500μ, 300μ, 200μ, 25μ or thinner.  
      In  FIG. 3 a  semiconductor die  300  manufactured from a silicon wafer, for example, provides the substrate. In this example, the method is used to grow the array  150  on a backside  310  of the semiconductor die  300 . A heat spreader  400  used to distribute heat from a semiconductor die to a heat sink in a semiconductor package provides the substrate in  FIG. 4 . As shown, the array  150  can be grown by the method on either the surface  410  that faces the semiconductor die, or on the surface  420  that faces the heat sink, or both. The array  150  can also be grown on a heat sink  500 , as illustrated by  FIG. 5 .  
       FIGS. 6 and 7  illustrate exemplary further steps to the method of  FIG. 1 . In  FIG. 6 a  metal layer  600  is formed on the second end  170  of the array  150  so that the carbon nanotubes extend partially into the metal layer  600 . A suitable metal for the metal layer  600  is copper. The metal layer  600  can be formed, for instance, by sputtering, evaporation, or electroplating. It should be noted that the metal layer  600  is not meant to infiltrate the entire array  150  but only to encapsulate the very ends of the carbon nanotubes and to extend a short distance above the second end  170 . An appropriate thickness for the metal layer  600  will depend on the density of carbon nanotubes in the array  150  and the variation in their heights, but a minimum thickness for the metal layer  800  is on the order of 200 Å.  
      In some embodiments, forming the metal layer  600  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  600  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  170  of the array  150 . Wetting and protection layers are described in more detail in U.S. Non-Provisional Patent Application Number 11/107,599 filed on Apr. 14, 2005 and titled “Nanotube Surface Coatings for Improved Wettability,” incorporated herein by reference in its entirety.  
      As shown in  FIG. 7 , the metal layer  600  can be polished to increase the smoothness of the surface. Polishing the metal layer  600  can comprise chemical mechanical polishing (CMP) which also serves to planarize the surface. Copper is a good choice for the metal layer  600 , in those embodiments that include CMP of the metal layer  600  in that CMP of copper has been refined in the semiconductor processing arts. In some embodiments, polishing the metal layer  600  continues until the second end  170  of the array  150  is exposed, while in other embodiments polishing is discontinued before that point is reached, as shown in  FIG. 7 .  
      As shown in  FIG. 8 , instead of forming and polishing a metal layer  600 , in other embodiments a thermal pad with a smooth surface is obtained by attaching a foil  800  to the array  150 . Attaching the foil  800  can include forming an attachment layer  810  on the second end  170  of the array  150  so that the carbon nanotubes extend partially into the attachment layer  810 . Ideally, the attachment layer  810  is formed of a low melting point metal or eutectic alloy such as indium, tin, bismuth, or a solder such as tin-silver, tin-lead, lead-silver, gold-germanium, or tin-antimony. The attachment layer  810  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  810 .  
      Copper and silver foils are examples of suitable foils  800 . The foil  800  can be joined to the attachment layer  810  by heating the foil  800  while in contact with the attachment layer  810  to briefly melt the attachment layer  810  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  810  by cleaning the attachment layer  810  with an acid such as hydrochloric acid prior to attaching the foil  800 .  
      Each of the thermal pads shown in  FIGS. 1-8  is characterized by an array  150  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. 9 and 10 , some or all of the interstitial space is filled.  
      For example, in  FIG. 9  the interstitial space is filled by a matrix material  900 . Examples of matrix materials include metals and polymers. The interstitial space of the array  150  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  150  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.  
       FIG. 10  shows the interstitial space of the array  150  partially filled with a base metal layer  1000  that surrounds the carbon nanotubes at the first end  160  of the array  150  but otherwise leaves the interstitial space empty. The base metal layer  1000  can be formed of a metal such as copper by electroplating with the catalyst layer  140  serving as an electrode. The base metal layer  1000 , like the matrix material  900 , is advantageous for further securing the array  150  to the catalyst layer  140 . The base metal layer  1000  both provides this advantage while still leaving much of the interstitial space empty for greater flexibility of the thermal pad. It should be understood that the matrix material  900 , or base metal layer  1000 , can be applied to any of the embodiments taught with respect to  FIGS. 1-8 .  
       FIG. 11  illustrates yet another variation on the method of forming a thermal pad. In this example, the catalyst layer  140  is patterned, prior to forming the array  150 , so that the carbon nanotubes of the array  150  grow in columns or bundles  1100 . The catalyst layer  140  can be patterned, for example, by conventional masking techniques known to the semiconductor processing arts. Patterning the catalyst layer  140  to produce the bundles  1100  can be useful for those thermal pads that do not have a top layer such as metal layer  600  or foil  800 . When the second end  170  of the array  150  of such a thermal pad is joined to a surface, the taller bundles  1100 , because of the spaces between the bundles  1100 , are able to bend until the shorter bundles  1100  also contact the surface. In a similar manner, bundles  100  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.  
      It should be noted that a continuous catalyst layer  140 , 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 Number 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  150  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  150  to longer at the edges. Likewise, a greater density of carbon nanotubes can be grown in areas of the array  150  in order to match the greater density to hot spots on the heat source.  
       FIGS. 12 and 13  illustrate still another variation on the method of forming a thermal pad. In this example, spacers  1200  are placed over the planar surface  120  of the substrate  110  before the array  150  is formed. In some embodiments, the spacers  1200  are placed on the catalyst layer  140  as shown in  FIG. 12 . Subsequently, the array  150  is formed, as shown in  FIG. 13 . Preferably, the array  150  is grown until a height of the array  150  exceeds a height of the spacers  1200 . A thermal pad including spacers  1200  can be advantageous during assembly of the thermal pad within a device, package, or other structure. Not only can the spacers  1200  provide an appropriate spacing between two objects such as a heat source and a heat sink, but the spacers  1200  can also prevent damage to the carbon nanotubes of the array  150  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 Number 11/124,005 noted above.  
       FIG. 14  illustrates that the method can also be used to provide an array  150  on both surfaces of a foil  800 . In these embodiments the method can be applied to one surface and then the other, or to both surfaces simultaneously. Additionally, each of the several layers  125 ,  130 ,  140  can be formed first on one surface and then on the other, while the two arrays  150  are then grown simultaneously. A thermal pad formed by this method advantageously includes approximately twice the thickness of carbon nanotubes after an equivalent processing time.  
      As the foil  800  requires some form of support, a frame (not shown) can be used, for example, to support the foil  800  having a catalyst layer  140  on both surfaces within a reaction chamber while arrays  150  of carbon nanotubes are synthesized on both surfaces. Similarly, as noted above in connection with  FIG. 4 , arrays  150  can be formed on multiple surfaces of other substrates such as the heat spreader  400 . In some embodiments multiple arrays  150  on a substrate are formed sequentially while in other embodiments the arrays  150  are formed simultaneously.  
      Another variation on the method performs the steps in a continuous fashion on the foil  800 . In these embodiments the foil  800  is initially wound on a spool. One end of the foil  800  is fed into a guide that provides support to the foil  800  while a transport mechanism carries the foil  800  through a series of sequential processes to form the various layers  125 ,  130 ,  140 , the array  150 , and any subsequent layers such as attachment layer  810 . This variation can be used to form the array  150  on only one side of the foil  800  or both sides, as in  FIG. 14 . The foil  800 , once fully processed, can be sectioned to form individual thermal pads or wound onto another spool. In other embodiments, only the layers  125 ,  130 ,  140  are formed on the foil  800  in the described manner, then the foil  800  is cut into sections or coupons, and these sections or coupons are individually or batch processed to form arrays  150  thereon.  
       FIG. 15  shows yet another alternate substrate for carrying out the method. In  FIG. 15 a  lead frame  1500  serves as the substrate. The lead frame  1500  includes a die bonding pad  1510  and support fingers  1520  that attach the die bonding pad  1510  to the remainder of the lead frame  1500  which can include a plurality of other identical die bonding pads  1510 . Thus, an array  150  can be formed on each pad  1510  of the lead frame  1500  by the method described above. In some embodiments, the lead frame  1500  is made of oxygen free high conductivity copper. A suitable thickness for a lead frame  1500  is about 250μ, though thinner and thicker ones can be used. After processing to form the array  150 , the die bonding pad  1510  with the array  150  thereon can be separated from the remainder of the lead frame  1500  by detaching the pad  1510  from the support fingers  1520 . In other embodiments, the die bonding pad  1510  is supported on a pedestal during processing and the pedestal heats the die bonding pad  1510  from beneath, for example, by inductive heating. It will be appreciated that these same heating techniques can also be applied to other embodiments described herein.  
      Various steps involved in forming the layers on the die bonding pads  1510  can require elevated temperatures. In some embodiments, an electric current, on the order of tens of amps, is applied across the die bonding pad  1510  in order to heat the die bonding pad  1510  during various deposition steps such as forming the array  150 . The electric current can be applied to the die bonding pad  1510  through probes that contact either ends of die bonding pad  1510  or close by on the support fingers  1520 .  
       FIGS. 16-18  illustrate an exemplary arrangement of a plurality of lead frames  1500  within a furnace  1600  for chemical vapor processing (CVD) to produce arrays  150 .  FIG. 16  shows a cross-section through the furnace  1600 ,  FIG. 17  shows a cross-sectional view of the furnace  1600  taken along the line  17 - 17  in  FIG. 16 , and  FIG. 18  shows an enlarged view of a portion of  FIG. 17  to show the lead frames supported in a boat  1800 . An exemplary furnace  1600  is a 5-inch thermal CVD system configured such that a carbon-containing gas can enter from one end of the furnace  1600 , react to form the arrays  150  on the lead frames  1500 , and exit the opposite end of the furnace  1600 .  
       FIGS. 1-14  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  150  of nanotubes can have thicknesses ranging from a few microns to over 1 mm. In particular, the thickness of the arrays  150  can be between 0.1 mm and 2 mm. Some thermal pads are characterized by a second end  170  with exposed nanotubes. Other thermal pads are characterized by a capped second end  170  where the capping is achieved with either an attached thin substrate  200  or foil  800 , or a metal layer  600  that is either unfinished, polished, or polished and planarized. Additionally, any of these thermal pads can include carbon nanotubes grown in bundles  1100 , and any can include spacers  1200 .  
      Any of these thermal pads can include a matrix material  900  that fills the interstitial space between the ends  160 ,  170  of the array  150 . Similarly, any can include a base metal layer  1000  that only partially fills the interstitial space of the array  150  around the carbon nanotubes at the first end  160 . 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.  
      Some thermal pads are fixedly attached to inflexible substrates, such as heat spreaders, where the second end  170  of the array  150  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. With the exception of the thermal pad shown in  FIG. 14 , these thermal pads are characterized by a foil or thin substrate attached to the first end  160 . The thermal pad of  FIG. 14  is characterized by a foil  800  between two arrays  150  where each array  150  presents a second end  170  with exposed nanotubes.  
      A thermal pad having a second end  170  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  170  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.  
      A thermal pad having a second end  170  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.  
      Either end of a thermal pad that comprises a thin substrate  200 , a foil  800 , an unfinished metal layer  600  ( FIG. 6 ), or a polished metal layer  600  ( FIG. 7 ) 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  600 . 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.  
      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.  
      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.