US 20070116626 A1
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.
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.
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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.
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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.
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.
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.
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.
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.
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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
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It should be noted that a continuous catalyst layer 140, as shown for example in
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
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
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.
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
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 (
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.