Patent Publication Number: US-2007116957-A1

Title: Carbon nanotube thermal pads

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      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 Producing Free-Standing Carbon Nanotube Thermal Pads” (attorney docket number PA3336US). This application is also 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 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  
      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  
      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 3000W/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 free-standing thermal pad comprises a sheet of carbon nanotubes having first and second opposing sides, and a first surface layer disposed on the first side of the sheet of carbon nanotubes. The first surface layer includes a first metal layer having a thickness of less than 500 microns, and an intermediate layer attaching the first metal layer to the first side of the sheet. In this thermal pad the carbon nanotubes are generally aligned perpendicular to the sheet. The free-standing thermal pad can further comprising a second surface layer disposed on the second side of the sheet of carbon nanotubes.  
      Another exemplary free-standing thermal pad comprises an array of carbon nanotubes having first and second opposing sides, the carbon nanotubes being generally aligned perpendicular to the sides of the array and disposed in discrete bundles, the bundles being separated by unfilled interstitial space, and a first surface layer disposed on the first side of the array of carbon nanotubes and having a thickness of less than 500 microns. In some of these embodiments, the thermal pad further comprises a base metal layer around the carbon nanotubes at the first side of the array, and the interstitial space between the bundles is unfilled between the base metal layer and the second side of the array.  
      An exemplary thermal pad comprises a substrate having first and second planar surfaces, a first array of carbon nanotubes disposed on the first planar surface, the carbon nanotubes being generally aligned perpendicular to the first array, and a second array of carbon nanotubes disposed on the second planar surface, the carbon nanotubes being generally aligned perpendicular to the second array.  
      Still another free-standing thermal pad consists of a sheet of carbon nanotubes generally aligned perpendicular to the sheet, and a metal layer disposed over one side of the sheet, the ends of the carbon nanotubes extending partially into the metal layer. In some embodiments, the metal layer comprises copper. The metal layer can also include a planar surface.  
    
    
     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 .  
       FIG. 19  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  
       FIG. 20  shows a cross-sectional view of the thermal pad of  FIG. 19  after separation from the substrate.  
       FIG. 21  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.  
       FIG. 22  shows a cross-sectional view of the thermal pad of  FIG. 21  after separation from the substrate.  
       FIG. 23  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.  
       FIG. 24  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.  
       FIG. 25  shows a cross-sectional view of the thermal pad of  FIG. 24  after separation from the substrate.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention provides carbon nanotube-based thermal pads, both free-standing and supported on a thin substrate such as a foil, a thin metal sheet, or the surface of a component of a device. Each thermal pad is characterized by an array of generally aligned carbon nanotubes, forming a sheet, and having a direction of alignment that is essentially perpendicular to the major surfaces of the thermal pad. 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 No. 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  as opposed to the prior embodiments where the array  150  forms a continuous sheet of carbon nanotubes. 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  1100  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 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  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.  
       FIG. 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 No. 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 .  
       FIG. 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 .  
       FIG. 19-25  are directed to further methods for forming thermal pads where the substrate on which the thermal pad is formed is ultimately separated from the thermal pad rather than becoming an integral part of the thermal pad as in the prior embodiments. As used herein, a “free-standing” thermal pad is defined as a sheet-like structure having two opposing external surfaces and including an array of vertically aligned carbon nanotubes having two opposing sides. A free-standing thermal pad, as used herein, can have one of three basic structures. In a first structure, both sides of the carbon nanotube array define the external surfaces of the thermal pad. In a second structure, one side of the carbon nanotube array defines one external surface of the thermal pad and a surface layer over the opposing side of the array defines the other external surface of the thermal pad. In a third structure, surface layers over both sides of the array define both external surfaces of the thermal pad. In the second and third structures, the thickness of any surface layer, as measured from the side of the array to the external surface, is less than 1 mm and preferably less than about 500μ.  
       FIG. 19  and  20  illustrate an exemplary method of forming a free-standing thermal pad. As shown by  FIG. 19 , a substrate  1910  with a generally planar surface  1920  is initially provided. Next, a separation layer  1930  is formed on the planar surface  1920 . The method can also include forming an optional barrier layer  1940  either before or after forming the separation layer  1930 . An optional interface layer  1950  is formed over the separation layer  1930 , and over the barrier layer  1940 , if present.  
      Next, a catalyst layer  1960  is formed. The catalyst layer  1960  can be formed either directly on the separation layer  1930 , on the barrier layer  1940 , or on the interface layer  1950 . After the catalyst layer  1960  has been formed, an array  1970  of carbon nanotubes is formed on the catalyst layer  1960 . The array  1970  is formed such that the carbon nanotubes are generally aligned in a direction  1975  perpendicular to the planar surface  1920 . The array  1970  includes a first end  1980  attached to the catalyst layer  1970  and a second end  1990  opposite the first end  1970 .  
      Examples of a suitable substrate  1910  include polished silicon and gallium arsenide wafers. Either can provide an atomically smooth planar surface  1920  on which to form the successive layers  1930 - 1970 . An example of a suitable separation layer  1930  is nickel oxide. A nickel oxide separation layer  1930  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  1930  is about 100 Å.  
      Another suitable separation layer  1930 , where the substrate  1910  is gallium arsenide, is aluminum arsenide. An aluminum arsenide separation layer  1930  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  1930  by deposition, the separation layer  1930  can be formed by ion implantation into the substrate  1910 . For example, hydrogen ions can be implanted into silicon to form a silicon hydride layer than can readily delaminate from the silicon.  
      The purpose of the barrier layer  1940  is to prevent diffusion between the substrate  1910  and/or the separation layer  1930  and the catalyst layer  1960 . Preventing such diffusion is desirable in those embodiments where either the substrate  1910  or the separation layer  1930  includes one or more elements that can poison the catalyst of the catalyst layer  1960  and prevent nanotube growth. It will be appreciated that barrier layer  1940  is essentially the same in these embodiments as barrier layer  125  described in detail above and can be formed in these embodiments of the materials and by the methods provided.  
      The interface layer  1950  is provided, where needed, to improve the catalyst layer  1960  which, in turn, provides for higher quality nanotubes characterized by higher wall crystallinities and fewer defects. Interface layer  1950  is essentially the same in these embodiments as interface layer  130  described in detail above. Likewise, catalyst layer  1960  and array  1970  are essentially the same in these embodiments as catalyst layer  140  and array  150  described in detail above. Both the interface layer  1950 , the catalyst layer  1960 , and the array  1970  can be formed in these embodiments of the materials and by the methods provided above for interface layer  130 , catalyst layer  140 , and the array  150 .  
      After the carbon nanotube array  1970  has been formed, the thermal pad is removed from the substrate  1910  at the separation layer  1930 , as shown in  FIG. 20 . It will be appreciated that the surface of the thermal pad, after release from the substrate  1910 , will be essentially as smooth as the surface of the substrate  1910  on which the array  1970  was formed, and therefore can be atomically smooth, or nearly so. As described below, further processing of the array  1970  can occur either before or after separation from the substrate  1910 .  
      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  1970  and the separation layer  1930 . In these embodiments, after the array  1970  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  1970  to spontaneously detach, or to detach upon the application of very little force.  
      An adhesive tape can be applied to the second end  1990 , in some embodiments, to both pull the array  1970  away from the separation layer  1930  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.  
      Alternatively, the separation layer  1930  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  1930 . 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  1930  comprises a water-soluble salt.  
      As shown in  FIG. 21 , another exemplary method includes a surface layer  2100  intended to become part of the finished thermal pad after separation from the substrate  1910 . In  FIG. 21 , the surface layer  2100  can be a thin layer of copper, for example. The particular material for the surface layer  2100  can be chosen with respect to the intended use of the thermal pad. For instance, a metal for the surface layer  2100  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  2100  where the thermal pad is to be attached to a silicon surface. Nickel and aluminum can be used for the surface layer  2100  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).  
      In the example of  FIG. 21 , compared to that in  FIG. 19  and  20 , the surface layer  2100  is substituted for the separation layer  1930  and the thermal pad is separated from the substrate  1910  by dissolving the substrate  1910 , as shown in  FIG. 22 . For example, where the substrate  1910  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  2100  can also be added into the embodiment illustrated by  FIG. 19 . For instance, the surface layer  2100  can be included between the separation layer  1930  and the catalyst layer  1960 . In some of these embodiments, the surface layer  2100  can also serve as a barrier layer  1940 .  
      Although the method of  FIG. 21  and  22  can produce a free-standing thermal pad, in some embodiments the free-standing thermal pad is never realized. Instead, the second end  1990  is attached to a surface of another object such as a foil or a thermal management aid prior to the dissolution of the substrate  1910 . Methods for attaching the second end  1990  to other objects are discussed further below.  
      In some of the embodiments represented generally by  FIG. 19-22 , the substrate  1910  is diced into sections or coupons before the thermal pad is separated from the substrate  1910 .  FIG. 23  illustrates a top view of a wafer  2300 , including an array  1970 , and coupons  2310  produced by dicing the wafer  2300 . With respect to the embodiments of  FIG. 19  and  20 , dicing can occur prior to separating the thermal pad from the substrate  1910 . With respect to the embodiments of  FIG. 21  and  22 , dicing can occur before the substrate  1910  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  1910  other than a semiconductor wafer can also be segmented prior to separating the substrate  1910  from the thermal pad or before dissolving the substrate  1910 . Metal substrates  1910  can be die cut, for example.  
      Yet another method for forming free-standing carbon nanotube thermal pads is illustrated in  FIG. 24  and  25 . In  FIG. 24  an array  1970  is formed on a substrate  1910  in a manner analogous to that shown by  FIG. 19  and  21 , but without either the separation layer  1930  or the surface layer  2100 . After the array  1970  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  1980  of the array  1970 . 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  1960 .  
      An example of such a gas is a mixture of water vapor and hydrogen gas. In one embodiment, the array  1970  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  1910 , as shown in  FIG. 25 .  
      The array  1970  in any of the above embodiments shown in  FIG. 19-25  can be further processed either prior to, or after, release from the substrate  1910  according to the methods illustrated above with respect to  FIGS. 6-13 .  
       FIGS. 1-14 , and  20 ,  22 , and  25  as shown or as further modified by the processes of  FIGS. 6-13 , also represent different embodiments of finished thermal pads. The thermal pads are generally thin in a vertical dimension, direction  155  in  FIG. 1 , and bounded by two generally parallel surfaces of essentially arbitrary dimensions that are perpendicular to the vertical dimension. One advantage of these thermal pads is that the width, depth, and overall shape of the thermal pads (i.e., as viewed normal to the two parallel surfaces) can be established through well known manufacturing techniques of the semiconductor industry such as masking and etching, or by cutting the desired shape from a larger sheet. Exemplary thermal pads have surface areas measured perpendicular to the vertical dimension ranging from about 1 mm×1 mm, or less, to over 6″×6″. Arrays of nanotubes can have thicknesses ranging from a few microns to over 1 mm. A preferable thickness range is from 0.1 mm to 2 mm.  
      In the above discussion, the arrays have been described as having two ends. As an array forms a sheet with two sides within a thermal pad, it will be understood that an end of the array is used herein interchangeably with a side of a sheet. Either end of an array may form a surface of a thermal pad, or either or both ends can be capped so that the ends are not exposed and another material defines the surface. In those thermal pads characterized by a surface other than an exposed end of the array, the material that defines the surface can be a thin substrate or foil, or a metal layer that is either unfinished, polished, or polished and planarized. Additionally, any of these thermal pads can include carbon nanotubes grown in bundles, where the bundles are separated from each other to allow the bundles to bend when compressed. Also, any of these thermal pads can include spacers.  
      Any of these thermal pads can also include a matrix material that fills the interstitial space between the ends of the array. Similarly, any can include a base metal layer that only partially fills the interstitial space of the array around the carbon nanotubes at the first end. 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 an inflexible substrate, such as a heat spreader, where the free surface of the thermal pad 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.  
      A thermal pad having exposed nanotubes as one surface can be joined by that surface to a surface of an object through the use of a low melting point metal or eutectic alloy or a solder. One advantage of this method of joining the thermal pad to the surface of the other object is that neither surface is required 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 exposed nanotubes as one surface 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 application of pressure and/or 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.  
      In those thermal pads where a surface is defined by a thin substrate, a foil, an unfinished metal layer, or a polished metal layer, that surface can be joined to a surface of another object in several ways. One method is to join the two surfaces with a metal having a melting point below that of the materials that define the opposing surfaces. For example, silver can be used to join a copper heat spreader with a palladium metal layer of a thermal pad. 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 thermal pad 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 surface of the thermal pad 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.