U.S. patent application number 11/433662 was filed with the patent office on 2007-05-24 for methods for forming carbon nanotube thermal pads.
This patent application is currently assigned to Molecular Nanosystems, Inc.. Invention is credited to Gang Gu, Lawrence S. Pan, Jim Protsenko, Srinivas Rao.
Application Number | 20070116626 11/433662 |
Document ID | / |
Family ID | 38053743 |
Filed Date | 2007-05-24 |
United States Patent
Application |
20070116626 |
Kind Code |
A1 |
Pan; Lawrence S. ; et
al. |
May 24, 2007 |
Methods for forming carbon nanotube thermal pads
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.
Inventors: |
Pan; Lawrence S.; (Los
Gatos, CA) ; Rao; Srinivas; (Saratoga, CA) ;
Protsenko; Jim; (San Jose, CA) ; Gu; Gang;
(Palo Alto, CA) |
Correspondence
Address: |
CARR & FERRELL LLP
2200 GENG ROAD
PALO ALTO
CA
94303
US
|
Assignee: |
Molecular Nanosystems, Inc.
|
Family ID: |
38053743 |
Appl. No.: |
11/433662 |
Filed: |
May 11, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60680262 |
May 11, 2005 |
|
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|
60691673 |
Jun 17, 2005 |
|
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60709611 |
Aug 19, 2005 |
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Current U.S.
Class: |
423/447.1 ;
257/E23.11 |
Current CPC
Class: |
H01L 2924/0002 20130101;
H01L 23/373 20130101; H01L 21/4871 20130101; H01L 2924/0002
20130101; H01L 2924/00 20130101 |
Class at
Publication: |
423/447.1 |
International
Class: |
D01F 9/12 20060101
D01F009/12 |
Claims
1. A method of forming a thermal pad comprising: providing a
substrate having a thickness of less than 500.mu. 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.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] 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.
[0003] 2. Description of the Prior Art
[0004] 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-.degree.K and higher at room temperature have been
reported.
[0005] 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.
[0006] Therefore, what is needed are better methods for attaching
heat sinks, sources, and spreaders that provides both mechanical
integrity and improved thermal conductivity.
SUMMARY
[0007] An exemplary method of forming a thermal pad comprises
providing a substrate having a thickness of less than 500.mu. 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.
[0008] 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 500.mu., less than 250.mu., or less than
100.mu.. 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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
[0013] 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.
[0014] FIG. 12 shows a cross-sectional view of a partially
completed thermal pad according to an exemplary embodiment of the
invention.
[0015] 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.
[0016] FIG. 14 shows a cross-sectional view of still another
thermal pad according to an exemplary embodiment of the
invention.
[0017] 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.
[0018] 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.
[0019] FIG. 17 shows a cross-sectional view of the lead frames and
furnace of FIG. 16 taken along the line 17-17.
[0020] FIG. 18 shows an enlarged view of a portion of the
cross-sectional view of FIG. 17.
DETAILED DESCRIPTION OF THE INVENTION
[0021] 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.
[0022] Some thermal pads are characterized by at least one, and in
some instances, two very smooth surfaces. A thermal pad with a
sufficiently smooth surface can adhere to another very smooth
surface, such as the backside surface of semiconductor die, much
like two microscope slides will adhere to each other. Surfaces of
thermal pads, whether very smooth or not, can also be attached to
an opposing surface with a metal layer, for example with solder,
indium, or silver. Advantageously, some thermal pads are also
characterized by a degree of flexibility and pliability. This can
make it easier to work with the thermal pads in assembly operations
and allows the thermal pads to conform to opposing surfaces that
are curved or irregular.
[0023] 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 .ANG., and more preferably 100 .ANG.. 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.
[0024] 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
.ANG., and more preferably 100 .ANG. 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.
[0025] 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.
[0026] 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.
[0027] 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
500.mu., 300.mu., 200.mu., 25.mu. or thinner.
[0028] 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.
[0029] 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 .ANG..
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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 250.mu., 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.
[0044] 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.
[0045] 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.
[0046] 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.times.1 mm,
or less, to over 6''.times.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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.degree. 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.
[0051] 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.
[0052] 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.
[0053] 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.
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