US20070116626A1

US20070116626A1 - Methods for forming carbon nanotube thermal pads

Info

Publication number: US20070116626A1
Application number: US11/433,662
Authority: US
Inventors: Lawrence Pan; Srinivas Rao; Jim Protsenko; Gang Gu
Original assignee: Molecular Nanosystems Inc
Current assignee: Molecular Nanosystems Inc
Priority date: 2005-05-11
Filing date: 2006-05-11
Publication date: 2007-05-24

Abstract

Methods for forming thermal pads including arrays of vertically aligned carbon nanotubes are provided. The thermal pads are formed on various substrates, including foils, thin self-supporting polished metals, semiconductor dies, heat management aids, and lead frames. The arrays are growth from a catalyst layer disposed on the substrate. Forming the array can include leaving the ends of the nanotubes unfinished, attaching a foil thereto, or coating the ends with a metal layer. The metal layer coating can then be polished to a desired smoothness. The array can be filled with a matrix material, only partially filled, or left unfilled. Where the substrate is a foil, the method can be a continuous process where foil is taken from a roll and fed through a series of formation steps. Where the substrate is a lead frame, heating can be generated by applying an current to a pad of the lead frame.

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'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.

Claims

1. A method of forming a thermal pad comprising:

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 such that the carbon nanotubes are generally aligned in a direction perpendicular to the planar surface, the array characterized by a first end attached to the catalyst layer and a second end opposite the first end.

2. The method of claim 1 wherein the substrate includes copper.

3. The method of claim 1 wherein the substrate includes silicon.

4. The method of claim 1 wherein providing the substrate includes supporting the substrate with a frame.

5. The method of claim 1 wherein providing the substrate includes feeding a foil from a roll into a guide and using a transport mechanism to move the foil along the guide.

6. The method of claim 1 further comprising forming an interface layer on the substrate before forming the catalyst layer.

7. The method of claim 6 wherein forming the interface layer includes depositing aluminum oxide.

8. The method of claim 6 further comprising forming a barrier layer on the substrate before forming the interface layer.

9. The method of claim 1 further comprising infiltrating a matrix material into the array to fill an interstitial space thereof between the first and second ends.

10. The method of claim 9 wherein infiltrating the matrix material includes injection molding a polymer to fill the interstitial space.

11. The method of claim 1 further comprising forming a base metal layer around the carbon nanotubes at the first end of the array such that an interstitial space of the array between the base metal layer and the second end of the array remains unfilled.

12. The method of claim 1 further comprising forming a metal layer on the second end of the array, wherein the carbon nanotubes extend at least partially into the metal layer.

13. The method of claim 12 further comprising polishing the metal layer.

14. The method of claim 12 wherein forming the metal layer includes coating the ends of the carbon nanotubes at the second end of the array with a wetting layer.

15. The method of claim 14 further comprising coating the ends of the carbon nanotubes with a protective layer over the wetting layer.

16. The method of claim 1 further comprising attaching a metal foil to the second end of the array.

17. The method of claim 16 wherein attaching the metal foil includes forming an attachment layer on the second end of the array.

18. The method of claim 1 wherein forming the catalyst layer includes patterning the catalyst layer to form a patterned catalyst layer, and wherein forming the array includes forming bundles of aligned carbon nanotubes on the patterned catalyst layer.

19. The method of claim 1 further comprising providing a spacer on the planar surface before forming the array.

20. The method of claim 1 further comprising

forming a second catalyst layer on a second planar surface of the substrate; and

forming a second array of carbon nanotubes on the second catalyst layer such that the carbon nanotubes are generally aligned in a direction perpendicular to the second planar surface.

21. A method of forming a thermal pad comprising:

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.

22. The method of claim 21 further wherein forming the array comprises heating the die bonding pad by applying a current thereto.

23. The method of claim 21 further comprising separating the die bonding pad from the lead frame.