U.S. patent application number 11/253290 was filed with the patent office on 2006-05-04 for carbonaceous composite heat spreader and associated methods.
Invention is credited to Chien-Min Sung.
Application Number | 20060091532 11/253290 |
Document ID | / |
Family ID | 32930080 |
Filed Date | 2006-05-04 |
United States Patent
Application |
20060091532 |
Kind Code |
A1 |
Sung; Chien-Min |
May 4, 2006 |
Carbonaceous composite heat spreader and associated methods
Abstract
A heat spreader including a plurality of carbonaceous particles
present in an amount of at least about 50% by volume of the heat
spreader. A non-carbonaceous material is also present in an amount
of at least about 5% by volume of the heat spreader, the
non-carbonaceous material including an element selected from the
group consisting of Cu, Al and Ag. In another aspect, the
carbonaceous particles may be sintered or fused directly to one
another. The heat spreader can be incorporated into a cooling unit
for transferring heat away from a heat source, which includes a
heat sink with the heat spreader disposed in thermal communication
with both the heat sink and the heat source.
Inventors: |
Sung; Chien-Min; (Tansui,
TW) |
Correspondence
Address: |
THORPE NORTH & WESTERN, LLP.
8180 SOUTH 700 EAST, SUITE 200
SANDY
UT
84070
US
|
Family ID: |
32930080 |
Appl. No.: |
11/253290 |
Filed: |
October 17, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10453469 |
Jun 2, 2003 |
6984888 |
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11253290 |
Oct 17, 2005 |
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10270018 |
Oct 11, 2002 |
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10453469 |
Jun 2, 2003 |
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Current U.S.
Class: |
257/720 ;
257/E23.11; 257/E23.111; 257/E23.112 |
Current CPC
Class: |
H01L 23/3733 20130101;
H01L 23/3732 20130101; H01L 2924/0002 20130101; H01L 23/373
20130101; H01L 2924/0002 20130101; H01L 2924/00 20130101; H01L
2924/01322 20130101 |
Class at
Publication: |
257/720 |
International
Class: |
H01L 23/34 20060101
H01L023/34 |
Claims
1. A heat spreader comprising: a plurality of carbonaceous
particles present in an amount of at least about 50% by volume of
the heat spreader; and a non-carbonaceous material present in an
amount of at least about 5% by volume of the heat spreader, said
non-carbonaceous material including an element selected from the
group consisting of Cu, Al and Ag.
2. The heat spreader of claim 1, wherein the carbonaceous particles
are present in an amount of at least about 80% by volume of the
heat spreader.
3. The heat spreader of claim 2, wherein the carbonaceous particles
are present in an amount of at least about 90% by volume of the
heat spreader.
4. The heat spreader of claim 1, wherein the carbonaceous particles
are diamond particles.
5. The heat spreader of claim 1, wherein the non-carbonaceous
material consists essentially of either Cu, Al, or Ag.
6. The heat spreader of claim 1, wherein the non-carbonaceous
material includes a carbide forming element from about 1% w/w to
about 10% w/w of the non-carbonaceous material.
7. The heat spreader of claim 6, wherein the carbide forming
element is present in an amount of at least about 1% w/w.
8. The heat spreader of claim 7, wherein the carbide forming
element is a member selected from the group consisting of: Sc, Y,
Ti, Zr, Hf, V, Nb, Cr, Mo, Mn, Ta, W, Tc, Si, B, Al, and alloys
thereof.
9. The heat spreader of claim 8, wherein the carbide forming
element is a Cu--Mn alloy.
10. The heat spreader of claim 1, wherein the non-carbonaceous
material is an alloy having a eutectic melting point below about
1100.degree. C.
11. The heat spreader of claim 1, wherein the non-carbonaceous
material is an alloy that wets the carbonaceous particles.
12. The heat spreader of claim 4, wherein the non-carbonaceous
material contains at least about 2% w/w of a carbide former.
13. The heat spreader of claim 1, wherein the heat spreader has a
thickness of greater than about 1 millimeter.
14. A cooling unit for transferring heat away from a heat source,
comprising: a heat sink; and a heat spreader as recited in claim 1
disposed in thermal communication with both the heat sink and the
heat source.
15. A cooling unit for transferring heat away from a heat source,
comprising: a heat sink; and a heat spreader comprising a mass of
diamond particles sintered directly to one another in thermal
communication with both the heat sink and the heat source.
16. The cooling unit of either of claims 14 or 15, wherein the heat
spreader is at least partially embedded in the heat source.
17. The cooling unit of either of claims 14 or 15, wherein the heat
spreader is at least partially embedded in the heat sink.
18. The cooling unit of claim 17, wherein the heat spreader is held
in the heat sink by a compression fit.
19. The cooling unit of claim 18, wherein the compression fit
holding the heat spreader in the heat sink is a thermally induced
compression fit.
20. The cooling unit of either of claims 14 or 15, wherein the heat
spreader is brazed to the heat sink.
21. The cooling unit of either of claims 14 or 15, wherein the heat
sink comprises a heat pipe having an internal working fluid.
22. The cooling unit of claim 21, wherein the heat spreader
protrudes through a wall of the heat pipe, and has a bottom surface
in direct contact with the working fluid of the heat pipe.
23. A method of making a heat spreader comprising the steps of:
providing a plurality of carbonaceous particles; and infiltrating
the plurality of carbonaceous particles with a non-carbonaceous
material as recited in any of claims 1 or 5-11, such that a heat
conducting mass is formed.
24. A method of cooling a heat source comprising the steps of:
providing a heat spreader as recited in any of claims 1-13; and
placing the heat spreader in thermal communication with both the
heat source and a heat sink.
25. The heat spreader of claim 4, wherein the diamond particles are
un-coated diamond particles.
Description
PRIORITY INFORMATION
[0001] This application is a continuation of copending U.S. patent
application Ser. No. 10/453,469, filed Jun. 2, 2003, which is a
continuation-in-part of U.S. patent application Ser. No.
10/270,018, filed Oct. 11, 2002, which are each herein incorporated
by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to carbonaceous composite
devices and systems that can be used to conduct or absorb heat away
from a heat source. Accordingly, the present invention involves the
fields of chemistry, physics, and materials science.
BACKGROUND OF THE INVENTION
[0003] Progress in the semiconductor industry has been following
the trend of Moore's Law that was proposed in 1965 by then Intel's
cofounder Gordon Moore. This trend requires that the capability of
integrated circuits (IC) or, in general, semiconductor chips double
every 18 months. Thus, the number of transistors on a central
processing unit (CPU) in 2002 may approach 100 million. As a result
of this densification of circuitry, line-width in 2002 narrowed to
0.18 micrometer and more advanced chips are using wires as thin as
0.13 micrometer. With this trend continuing, it is projected that
the seemingly impermeable "Point One" barrier, of 0.1 micrometer,
will be attained and surpassed in the next few years.
[0004] Along with such advances comes various design challenges.
One of the often overlooked challenges is that of heat dissipation.
Most often, this phase of design is neglected or added as a last
minute design before the components are produced. According to the
second law of thermodynamics, the more work that is performed in a
closed system, the higher entropy it will attain. With the
increasing power of a CPU, the larger flow of electrons produces a
greater amount of heat. Therefore, in order to prevent the
circuitry from shorting or burning out, the heat resulting from the
increase in entropy must be removed. Some state-of-the-art CPUs
have a power of about 60 watts (W). For example, a CPU made with
0.13 micrometer technology may exceed 100 watts. Current methods of
heat dissipation, such as by using metal (e.g., Al or Cu) fin
radiators, and water evaporation heat pipes, will be inadequate to
sufficiently cool future generations of CPUs.
[0005] Recently, ceramic heat spreaders (e.g., AlN) and metal
matrix composite heat spreaders (e.g., SiC/Al) have been used to
cope with the increasing amounts of heat generation. However, such
materials have a thermal conductivity that is no greater than that
of Cu, hence, their ability to dissipate heat from semiconductor
chips is limited.
[0006] A typical semiconductor chip contains closely packed metal
conductors (e.g., Al, Cu) and ceramic insulators (e.g., oxide,
nitride). The thermal expansion of metal is typically 5-10 times
that of ceramics. When the chip is heated to above 60.degree. C.,
the mismatch of thermal expansions between metal and ceramics can
create microcracks. The repeated cycling of temperature tends to
aggravate the damage to the chip. As a result, the performance of
the semiconductor will deteriorate. Moreover, when temperatures
reach more than 90.degree. C., the semiconductor portion of the
chip may become a conductor so the function of the chip is lost. In
addition, the circuitry may be damaged and the semiconductor is no
longer usable (i.e. becomes "burned out"). Thus, in order to
maintain the performance of the semiconductor, its temperature must
be kept below a threshold level (e.g., 90.degree. C.).
[0007] A conventional method of heat dissipation is to contact the
semiconductor with a metal heat sink. A typical heat sink is made
of aluminum that contains radiating fins. These fins are attached
to a fan. Heat from the chip will flow to the aluminum base and
will be transmitted to the radiating fins and carried away by the
circulated air via convection. Heat sinks are therefore often
designed to have a high heat capacity to act as a reservoir to
remove heat from the heat source.
[0008] Alternatively, a heat pipe may be connected between the heat
sink and a radiator that is located in a separated location. The
heat pipe contains water vapor that is sealed in a vacuum tube. The
moisture will be vaporized at the heat sink and condensed at the
radiator. The condensed water will flow back to the heat sink by
the wick action of a porous medium (e.g., copper powder). Hence,
the heat of a semiconductor chip is carried away by evaporating
water and removed at the radiator by condensing water.
[0009] Although heat pipes and heat plates may remove heat very
efficiently, the complex vacuum chambers and sophisticated
capillary systems prevent designs small enough to dissipate heat
directly from a semiconductor component. As a result, these methods
are generally limited to transferring heat from a larger heat
source, e.g., a heat sink. Thus, removing heat via conduction from
an electronic component is a continuing area of research in the
industry.
[0010] One promising alternative that has been explored for use in
heat spreaders is diamond-containing materials. Diamond can carry
away heat much faster than any other material. The thermal
conductivity of diamond at room temperature (about 2000 W/mK) is
much higher than either copper (about 400 W/mK) or aluminum (250
W/mK), the two fastest metal heat conductors commonly used.
Moreover, the thermal capacity of diamond (1.5 J/cm.sup.3) is much
lower than copper (17 J/cm.sup.3) or aluminum (24 J/cm.sup.3). The
ability for diamond to carry away heat without storing it makes
diamond an ideal heat spreader. In contrast to heat sinks, a heat
spreader acts to quickly conduct heat away from the heat source
without storing it. Table 1 shows various thermal properties of
several materials as compared to diamond (values provided at 300
K). TABLE-US-00001 TABLE 1 Thermal Thermal Conductivity Heat
Capacity Expansion Material (W/mK) (J/cm.sup.3 K) (1/K) Copper 401
3.44 1.64E-5 Aluminum 237 2.44 2.4E-5 Molybdenum 138 2.57 4.75E-5
Gold 317 2.49 1.43E-5 Silver 429 2.47 1.87E-5 Tungsten Carbide 95
2.95 0.57E-5 Silicon 148 1.66 0.258E-5 Diamond (IIa) 2,300 1.78
0.14E-5
[0011] In addition, the thermal expansion coefficient of diamond is
one of the lowest of all materials. The low thermal expansion of
diamond makes joining it with low thermally expanding silicon
semiconductor much easier. Hence, the stress at the joining
interface can be minimized. The result is a stable bond between
diamond and silicon that does not delaminate under the repeated
heating cycles.
[0012] In recent years diamond heat spreaders have been used to
dissipate heat from high power laser diodes, such as that used by
laser diodes to boost the light energy in optical fibers. However,
large area diamonds are very expensive; hence, diamond has not been
commercially used to spread the heat generated by CPUs. In order
for diamond to be used as a heat spreader, its surface must be
polished so it can make an intimate contact with the semiconductor
chip. Moreover, its surface may be metallized (e.g., by Ti/Pt/Au)
to allow attachment to a conventional metal heat sink by
brazing.
[0013] Many current diamond heat spreaders are made of diamond
films formed by chemical vapor deposition (CVD). One example of raw
CVD diamond films are now sold at over $10/cm.sup.2, and this price
may doubled when it is polished and metallized. This high price
would prohibit diamond heat spreaders from being widely used except
in those applications (e.g., high power laser diodes) where only a
small area is required or no effective alternative heat spreaders
are available. In addition to being expensive, CVD diamond films
can only be grown at very slow rates (e.g., a few micrometers per
hour); hence, these films seldom exceed a thickness of 1 mm
(typically 0.3-0.5 mm). However, if the heating area of the chip is
large (e.g., a CPU), it is preferable to have a thicker (e.g., 3
mm) heat spreader.
[0014] In addition to diamond products produced using CVD methods,
attempts have been made to form heat spreaders using a mass of
particulate diamond or "polycrystalline diamond" (PCD). Specific
examples of such devices are found in U.S. Pat. No. 6,390,181, and
U.S. Patent Application Publication No.2002/0023733, each of which
is incorporated herein by reference. Typically, a PCD product (or
"compact") is formed by processing diamond particles under
high-pressure, high-temperature (HPHT) conditions to thereby cause
the diamond particles to sinter or bond to each other and/or an
interstitial material. As a result, most PCD compacts have a
relatively small thickness due to the extreme pressures required by
the HPHT process. Because of the extremely high pressures used, the
mold or cavity used in producing PCD compacts has been limited to a
small thickness so that the mechanical equipment creating the
extremely high pressure can maintain the pressure and temperature
required. Such PCD compacts are of limited use in the field of heat
spreaders because of their limited physical capacity to transfer or
conduct heat.
[0015] As such, cost effective systems and devices that are capable
of effectively conducting heat away from a heat source, continue to
be sought through ongoing research and development efforts.
SUMMARY OF THE INVENTION
[0016] Accordingly, the present invention provides composite heat
spreaders that can be used to draw or conduct heat away from a heat
source. In one aspect, the heat spreader includes a plurality of
carbonaceous particles present in an amount of at least about 50%
by volume of the heat spreader. A non-carbonaceous infiltrant can
be present in an amount of at least about 5% by volume of the heat
spreader. The non-carbonaceous infiltrant can include an element
selected from the group consisting of Cu, Al and Ag.
[0017] In accordance with another aspect of the invention, the
carbonaceous particles can be present in an amount of at least
about 80% by volume of the heat spreader, and in another aspect can
be present in an amount of at least about 90% by volume of the heat
spreader. The carbonaceous particles can comprise diamond
particles.
[0018] In accordance with another aspect of the invention, the
infiltrant includes a carbide forming element and from about 1% w/w
to about 10% w/w of either Cu, Al, or Ag. The carbide forming
element can be a member selected from the group consisting of: Sc,
Y, Ti, Zr, Hf, V, Nb, Cr, Mo, Mn, Ta, W, Tc, Si, B, Al, and alloys
thereof. The carbide forming element can also be a Cu--Mn
alloy.
[0019] In accordance with another aspect of the invention, the
infiltrant can be an alloy having a eutectic melting point below
about 1100.degree. C. The infiltrant can be an alloy that wets the
carbonaceous particles.
[0020] In accordance with another aspect of the invention, the
diamond particles can be present in an amount of greater than about
50% by volume of the heat spreader, and the infiltrant can be
present in an amount of greater than about 5% by volume of the heat
spreader, and can contain at least about 2% w/w of a carbide
former.
[0021] In accordance with yet another aspect of the invention, a
cooling unit for transferring heat away from a heat source is
provided, and includes a heat sink and a heat spreader in
accordance with the heat spreader embodiments recited herein. The
heat spreader can be disposed in thermal communication with both
the heat sink and the heat source.
[0022] In accordance with yet another aspect of the invention, a
cooling unit for transferring heat away from a heat source is
provided, and includes a heat sink and a heat spreader comprising a
mass of diamond particles sintered directly to one another. The
heat spreader can be in thermal communication with both the heat
sink and the heat source.
[0023] In accordance with another aspect of the invention, the heat
spreader can be at least partially embedded in the heat source
and/or the heat sink.
[0024] In accordance with another aspect of the invention, the heat
spreader is held in the heat sink by a compression fit. The
compression fit holding the heat spreader in the heat sink can be a
thermally induced compression fit.
[0025] In accordance with another aspect of the invention, the heat
sink comprises a heat pipe having an internal working fluid. The
heat spreader can protrude through a wall of the heat pipe, and can
have a bottom surface in direct contact with the working fluid of
the heat pipe.
[0026] In accordance with still another aspect of the invention, a
method of making a heat spreader is provided and includes the steps
of providing a plurality of carbonaceous particles, and
infiltrating the plurality of carbonaceous particles with a
non-carbonaceous infiltrant according to aspects of the heat
spreaders as recited herein.
[0027] In accordance with yet another aspect of the invention, a
method of cooling a heat source is provided and includes the steps
of providing a heat spreader according to aspects of heat spreaders
recited herein, and placing the heat spreader in thermal
communication with both the heat source and a heat sink.
[0028] There has thus been outlined, rather broadly, various
features of the invention so that the detailed description thereof
that follows may be better understood, and so that the present
contribution to the art may be better appreciated. Other features
of the present invention will become clearer from the following
detailed description of the invention, taken with the accompanying
claims, or may be learned by the practice of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1a is a schematic view of a heat spreader in thermal
communication with a heat source and a heat sink in accordance with
an embodiment of the present invention;
[0030] FIG. 1b is a schematic view of a heat spreader in thermal
communication with a heat source and a heat sink in accordance with
another embodiment of the present invention; and
[0031] FIG. 1c is a schematic view of a heat spreader in thermal
communication with a heat source and a heat sink in accordance with
another embodiment of the present invention.
DETAILED DESCRIPTION
[0032] Before the present invention is disclosed and described, it
is to be understood that this invention is not limited to the
particular structures, process steps, or materials disclosed
herein, but is extended to equivalents thereof as would be
recognized by those ordinarily skilled in the relevant arts. It
should also be understood that terminology employed herein is used
for the purpose of describing particular embodiments only and is
not intended to be limiting.
[0033] It must be noted that, as used in this specification and the
appended claims, the singular forms "a," "an" and, "the" include
plural referents unless the context clearly dictates otherwise.
Thus, for example, reference to "a diamond particle" includes one
or more of such particles, reference to "an interstitial material"
includes reference to one or more of such materials, and reference
to "the particle" includes reference to one or more of such
particles.
[0034] Definitions
[0035] In describing and claiming the present invention, the
following terminology will be used in accordance with the
definitions set forth below.
[0036] As used herein, "particle" and "grit" may be used
interchangeably, and when used in connection with a carbonaceous
material, refer to a particulate form of such material. Such
particles or grits may take a variety of shapes, including round,
oblong, square, euhedral, etc., as well as a number of specific
mesh sizes. As is known in the art, "mesh" refers to the number of
holes per unit area as in the case of U.S. meshes. All mesh sizes
referred to herein are U.S. mesh unless otherwise indicated.
Further, mesh sizes are generally understood to indicate an average
mesh size of a given collection of particles since each particle
within a particular "mesh size" may actually vary over a small
distribution of sizes.
[0037] As used herein, "substantial," or "substantially" refers to
the functional achievement of a desired purpose, operation, or
configuration, as though such purpose or configuration had actually
been attained. Therefore, carbonaceous particles that are
substantially in contact with one another function as though, or
nearly as though, they were in actual contact with one another. In
the same regard, carbonaceous particles that are of substantially
the same size operate, or obtain a configuration as though they
were each exactly the same size, even though they may vary in size
somewhat.
[0038] As used herein, "heat spreader" refers to a material which
distributes or conducts heat and transfers heat away from a heat
source. Heat spreaders are distinct from heat sinks which are used
as a reservoir for heat to be held in, until it can be transferred
away from the heat sink by another mechanism, whereas a heat
spreader does not retain a significant amount of heat, but merely
transfers heat away from a heat source.
[0039] As used herein, "heat source" refers to a device or object
having an amount of thermal energy or heat which is greater than
desired. Heat sources can include devices that produce heat as a
byproduct of their operation, as well as objects that become heated
to a temperature that is higher than desired by a transfer of heat
thereto from another heat source.
[0040] As used herein, "carbonaceous" refers to any material which
is made primarily of carbon atoms. A variety of bonding
arrangements or "allotropes" are known for carbon atoms, including
planar, distorted tetrahedral, and tetrahedral bonding
arrangements. As is known to those of ordinary skill in the art,
such bonding arrangements determine the specific resultant
material, such as graphite, diamond-like-carbon (DLC), or amorphous
diamond, and pure diamond. In one aspect, the carbonaceous material
may be diamond.
[0041] As used herein, "reactive element" and "reactive metal" may
be used interchangeably, and refer to an element, especially a
metal element that can chemically react with and chemically bond to
carbon by forming a carbide bond. Examples of reactive elements may
include without limitation, transition metals such as titanium (Ti)
and chromium (Cr), including refractory elements, such as zirconium
(Zr) and tungsten (W), as well as non-transition metals and other
materials, such as aluminum (Al). Further, certain non-metal
elements such as silicon (Si) may be included as a reactive element
in a brazing alloy.
[0042] As used herein "wetting" refers to the process of flowing a
molten metal across at least a portion of the surface of a
carbonaceous particle. Wetting is often due, at least in part, to
the surface tension of the molten metal, and may be facilitated by
the use or addition of certain metals to the molten metal. In some
aspects, wetting may aid in the formation of chemical bonds between
the carbonaceous particle and the molten metal at the interface
thereof when a carbide forming metal is utilized.
[0043] As used herein, "chemical bond" and "chemical bonding" may
be used interchangeably, and refer to a molecular bond that exerts
an attractive force between atoms that is sufficiently strong to
create a binary solid compound at an interface between the atoms.
Chemical bonds involved in the present invention are typically
carbides in the case of diamond superabrasive particles, or
nitrides or borides in the case of cubic boron nitride.
[0044] As used herein, "braze alloy" and "brazing alloy" may be
used interchangeably, and refer to an alloy containing a sufficient
amount of a reactive element to allow the formation of chemical
bonds between the alloy and a superabrasive particle. The alloy may
be either a solid or liquid solution of a metal carrier solvent
having a reactive element solute therein. Moreover, the term
"brazed" may be used to refer to the formation of chemical bonds
between a superabrasive particle and a braze alloy.
[0045] As used herein, "sintering" refers to the joining of two or
more individual particles to form a continuous solid mass. The
process of sintering involves the consolidation of particles to at
least partially eliminate voids between particles. Sintering may
occur in either metal or carbonaceous particles, such as diamond.
Sintering of metal particles occurs at various temperatures
depending on the composition of the material. Sintering of diamond
particles generally requires ultrahigh pressures and the presence
of a carbon solvent as a diamond sintering aid, and is discussed in
more detail below. Sintering aids are often present to aid in the
sintering process and a portion of such may remain in the final
product.
[0046] Concentrations, amounts, particle sizes, volumes, and other
numerical data may be expressed or presented herein in a range
format. It is to be understood that such a range format is used
merely for convenience and brevity and thus should be interpreted
flexibly to include not only the numerical values explicitly
recited as the limits of the range, but also to include all the
individual numerical values or sub-ranges encompassed within that
range as if each numerical value and sub-range is explicitly
recited.
[0047] As an illustration, a numerical range of "about 1 micrometer
to about 5 micrometers" should be interpreted to include not only
the explicitly recited values of about 1 micrometer to about 5
micrometers, but also include individual values and sub-ranges
within the indicated range. Thus, included in this numerical range
are individual values such as 2, 3, and 4 and sub-ranges such as
from 1-3, from 2-4, and from 3-5, etc. This same principle applies
to ranges reciting only one numerical value. Furthermore, such an
interpretation should apply regardless of the breadth of the range
or the characteristics being described.
[0048] The Invention
[0049] The present invention encompasses devices, systems, and
methods for transferring heat away from a heat source. Heat
spreaders made in accordance with the method of the present
invention generally contain a plurality of carbonaceous particles,
including embodiments in which carbonaceous particles are each
substantially in contact with one another. The plurality of
carbonaceous particles may either be bound together using an
interstitial material or by direct sintering or fusing of the
carbonaceous particles themselves into a mass.
[0050] In one aspect, a general process for making a heat spreader
having a high carbonaceous particle volume begins with the packing
of a first plurality of carbonaceous particles in a suitable mold.
Optionally, the first plurality of particles may each be
approximately the same mesh size. The specific size of these
particles may be up to about 18 mesh (1 mm) with sizes between
about 30 mesh (0.5 mm) and about 400 mesh (37 micrometers) being
typical. While the size of the particles may vary, the general
principle is recognized that larger carbonaceous particles provide
for a larger path having improved heat transfer characteristics
which approach that of a solid carbonaceous material, such as pure
diamond.
[0051] The particles are packed such that there is substantial
contact between particles. Each particle should be in contact with
at least one other particle in the packed group. Thus, there may be
groups of particles which are in contact with one another separate
from the remaining particles. In another aspect of the present
invention the contact between particles may be sufficient to
provide a continuous path to substantially all of the carbonaceous
particles in the heat spreader. The transfer of heat away from the
heat source is facilitated by the substantial particle-particle
contact. The particles can be packed so as to occupy most of the
volume and minimize the amount of empty void between particles.
[0052] In one aspect, particle packing that obtains the
above-recited goals may be achieved by packing carbonaceous
particles of different sizes in successive stages. For example,
larger carbonaceous particles are packed into a suitable mold. The
packing of the carbonaceous particles may be improved by settling
or otherwise compacting, e.g., agitated inside the mold by a
vibrator. A plurality of smaller carbonaceous particles may then be
added to fill the voids surrounding the larger carbonaceous
particles. Depending on the size of the smaller particles, the
smaller particles may need to be introduced from multiple sides of
the packed carbon material in order to fill most of the available
voids. The size of the smaller carbonaceous particles may vary.
Typically particles in the range of between about 1/3.sup.rd to
about 1/20.sup.th of the diameter of the larger diamond will
increase the packing efficiency. Particles which are between about
1/5.sup.th and about 1/10.sup.th may also be used, while particles
1/7.sup.th the diameter of the larger particles have been used with
good results. Using such successive packing stages, the volume
packing efficiency may reach greater than two-thirds. If necessary,
addition of even smaller carbonaceous particles may be performed to
increase the packing efficiency further. However, this successive
packing method will soon reach a point of diminishing returns as
the filling becomes more and more difficult while the increase in
packing efficiency becomes less and less. The packed particles made
in accordance with the above principles in mind will provide a
carbon volume content of between about 50% and about 80%.
[0053] In an alternative embodiment, the different sized
carbonaceous particles are mixed first and then packed together
prior to introduction of the interstitial material. This approach
allows for an increase in packing efficiency, however, some thermal
benefits may be sacrificed as a result of the larger particles not
being in intimate contact with other larger particles. Thus, heat
must cross a greater number of particle-particle interface
boundaries increasing the thermal resistance of the final heat
spreader.
[0054] In yet another alternative embodiment, the volume of diamond
may be increased by using uniformly shaped carbonaceous particles.
In particular, substantially cubic diamond particles are
commercially available, although other shapes could be used. The
cubic diamonds may be packed edge-to-edge to produce a layer, or
layers, of packed diamond particles with a diamond volume content
of up to about 90%-95%. The specific arrangement is unimportant and
the particles may be packed in ordered rows and columns or the rows
and columns may be staggered. In this embodiment, the arrangement
of diamond particles allows for substantially smaller volume of
void between particles without sintering the diamond particles
together. In addition, the thermal properties of the final
composite are improved if the particles are all oriented in the
same direction as opposed to random directions. The following
discussions of interstitial materials and processing apply to this
arrangement of packed diamond particles as to the above described
packed diamond particles.
[0055] In one aspect of the invention, an interstitial material may
be used in connection with the carbonaceous particles in order to
bond them together into a composite mass. However, by packing the
particles prior to introduction of the interstitial material as
recited above, the original particle-to-particle contact can be
maintained so that the packing efficiency far exceeds the
efficiency obtained by first mixing the carbonaceous particles with
an interstitial material and then consolidating by hot pressing. In
the latter case, the carbonaceous particles are likely to
constitute less than one-half of the device volume, as the
interstitial material tends to fill around diamond particles and
between them, thus completely separating many particles from one
another. In this case, heat must cross significant areas of
non-carbonaceous material.
[0056] Thus, in accordance with one aspect of the present
invention, the carbonaceous particles are packed before the
introduction of any non-carbonaceous materials as discussed above.
One factor to consider in designing a carbonaceous composite heat
spreader of the present invention is the thermal properties of the
composite at the interfaces between carbonaceous particles and the
interfaces between non-carbonaceous material and carbonaceous
particles. Empty voids and mere mechanical contact between
interfaces act as a thermal barriers. Although intimate contact of
carbonaceous particles along a significant portion of the surface
of the particles improves the thermal properties at these
boundaries, the result is somewhat inferior to that of pure
continuous carbonaceous material. Thus, it is desirable that a
substantial portion of the interfaces are more than mere mechanical
contact.
[0057] Accordingly, an interstitial material may be utilized having
specific characteristics suitable to achieving a particular
finished tool. In one aspect, the interstitial material may be
suitable to act as a carbon sintering aid under ultrahigh pressure
to sinter or actually fuse the carbonaceous particles together. In
another aspect of the present invention, an interstitial material
may be selected which chemically bonds the packed diamond particles
together.
[0058] The choice of interstitial material must account for the
thermal conductivity and heat capacity of the interstitial material
itself. A diamond compact heat spreader which contains material
having a low thermal conductivity will act as a limiting element
within the structure thus obviating some of the heat transfer
benefits of diamond. Therefore, an interstitial material which has
high thermal conductivity, low heat capacity, and provides for a
chemical bond with diamond greatly facilitates the heat transfer
across interface boundaries. Of course, a larger degree of
diamond-diamond contact will also improve the heat transfer
properties of the heat spreader.
[0059] The interstitial material for bonding or sintering of
carbonaceous particles may be provided in a number of ways
including infiltration, sintering and electro-deposition.
Infiltration occurs when a material is heated to its melting point
and then flows as a liquid through the interstitial voids between
particles. Sintering occurs when the interstitial material is
heated sufficient to cause neighboring particles of material to
melt near their edges and sinter neighboring particles together in
an essentially solid-state process. Thus, substantially no fluid
flow of the interstitial material would occur. Electro-deposition
involves depositing a metal heated above its melting point in
solution on the surface of the carbonaceous particles under an
electrical current.
[0060] Two basic categories of interstitial material include liquid
metal and molten ceramics. When bonding the carbonaceous particles
to produce a carbonaceous composite heat spreader the interstitial
material should contain at least one active element that will react
with carbon to form carbide. The presence of a carbide former aids
in the wetting of the diamond particles and causes the interstitial
material to be pulled into the interstitial voids by capillary
force. When sintering the carbonaceous particles to produce a
carbonaceous heat spreader, the interstitial material should act as
a sintering aid to increase the degree of carbon sintering and does
not necessarily contain a carbide former but rather contains a
carbon solvent.
[0061] In one aspect of the present invention, an interstitial
infiltrating alloy can be used as an infiltrant to bond the
carbonaceous particles into a substantially solid heat spreading
mass. As mentioned above, many interstitial materials may actually
hinder the transfer of heat through the heat spreader. For example,
interstitial materials that do not chemically bond with carbon, but
merely hold it mechanically can slow down the transfer of heat.
Further, many refractory materials that are good carbide formers
are poor heat conductors.
[0062] An additional consideration, when the carbonaceous material
is diamond, is that care must be taken in choosing an interstitial
material so as to avoid an infiltration or sintering temperature
that is high enough to damage the diamond. Therefore, in one aspect
of the invention, the interstitial material may be an alloy that
melts or sinters below about 1,100.degree. C. When heating above
this temperature, the time should be minimized to avoid excessive
damage to the diamond particles. Damage to the diamond particles
may also be induced internally due to cracking of the diamond from
the site of metal inclusions. Synthetic diamonds always contain a
metal catalyst (e.g., Fe, Co, Ni or its alloy) as inclusions. These
metal inclusions have high thermal expansion coefficients and they
can back-convert diamond into graphitic carbon. Hence, at high
temperature, diamond will crack due to the different thermal
expansion of metal inclusions or back-convert diamond to
carbon.
[0063] In accordance with the present invention, interstitial
materials may contain a diamond or carbon braze as a metal
infiltrant or silicon alloys as ceramic infiltrants. Moreover, the
infiltrant may be able to "wet" carbon so it can be wicked in the
interstitial of carbonaceous particles by capillary force. The
interstitial material substantially fills any of the remaining
voids between the packed carbonaceous particles. Common carbon
wetting agents include Co, Ni, Fe, Si, Mn, and Cr. When the
carbonaceous particles are to be chemically bonded together the
interstitial material may contain a carbide former which provides
for improved thermal properties at the boundaries between
particles. Such carbide formers include Sc, Y, Ti, Zr, Hf, V, Nb,
Cr, Mo, Mn, Ta, W, Tc, Si, B, Al, and alloys thereof.
[0064] Interstitial or infiltrating materials of the present
invention may include a component such as Ag, Cu, Al, Si, Fe, Ni,
Co, Mn, W, or their alloys or mixtures. Diamond or carbon brazes
include Fe, Co, or Ni alloys which exhibit wetting of the
carbonaceous particles. Alloys of these brazes may also contain a
carbide former such as Ti, Zr, or Cr. Ceramic silicon alloys may
contain Ni, Ti, or Cr. For example, Ni--Cr alloys, such as BNi2
(Ni--Cr--B) or BNi7 (Ni--Cr--P) are good infiltrants. Other
examples of effective infiltrants include Al--Si, Cu--Sn--Ti,
Ag--Cu--Ti, and Cu--Ni--Zr--Ti. Most carbonaceous interstitial
materials contain active elements (e.g., Cr, Ti) that not only bond
to carbon by forming carbide, but are also easily oxidized. Hence,
the introduction of interstitial materials should be performed in a
vacuum furnace or under the protection of an inert atmosphere.
[0065] The above carbonaceous composite heat spreaders can be
produced by at least partially filling in the pores or gaps among
carbonaceous particles by an interstitial material that can conduct
heat relatively fast. The interstitial material may be introduced
into the packed particles in a variety of ways. One way to provide
the interstitial material is by electro-deposition (e.g., Ag, Cu,
Ni) in a water solution. The metal is most often provided in an
acid solution and may be performed by those skilled in this art.
Various additional elements may also be added to lessen the surface
tension of the solution or to otherwise improve infiltration into
the voids.
[0066] In another aspect of the invention, a carbonaceous heat
spreader is provided that includes a plurality of carbonaceous
particles present in an amount of at least about 50% by volume of
the heat spreader. A non-carbonaceous infiltrant is present in an
amount of at least about 5% by volume of the heat spreader. The
non-carbonaceous infiltrant can include an element selected from
the group consisting of Cu, Al and Ag. In a further aspect of this
embodiment, the carbonaceous particles can be present in an amount
of at least about 80% by volume of the heat spreader, or at least
90% by volume of the heat spreader.
[0067] As in other aspects of the invention, the carbonaceous
particles can comprise diamond particles. The diamond particles can
be present in an amount of greater than about 50% by volume of the
heat spreader. The infiltrant, which can contain at least about 2%
w/w of a carbide former, can be present in an amount of greater
than about 5% by volume of the heat spreader.
[0068] In another aspect of the invention, a heat spreader is
provided that includes carbonaceous particles and a
non-carbonaceous infiltrant that may or may not chemically bond to
the carbonaceous particles. In this embodiment, the
non-carbonaceous infiltrant can be, for example, Cu, Al or Ag. By
processing the heat spreader under relatively high pressures, the
non-carbonaceous infiltrant increase the resulting heat spreading
capacity of the heat spreader while increasing the packing density
of the carbonaceous particles. For instance, in the case where the
carbonaceous particles are diamond, when the heat spreader is
processed at ultrahigh pressure the diamond grains can at least
partially crush at diamond-to-diamond contact points. As the
diamond particles crush, molten Cu, Al or Ag can be partially
injected into the diamond grains and thereby lead to an increase in
the diamond particle density.
[0069] As the resulting heat spreader includes a high concentration
of diamond particles, with substantially all of the voids between
diamond particles being filled with Cu, Al, or Ag, the heat
spreader can exhibit a high degree of thermal conductivity. Heat
spreaders made in accordance with this embodiment have been found
to exhibit thermal conductivity on the order of 11/2 to 2 times
that of pure copper. In addition, as Cu, Al and Ag are relatively
inexpensive materials, a heat spreader in accordance with this
embodiment can be made at commercially competitive costs. Also, as
Cu, in particular, has relatively low melting point (below about
1100.degree. C.), the process can be conducted at lower
temperatures and pressure that may otherwise be required in
conventional diamond PCD formation processes.
[0070] Some prior art diamond composite heat spreaders have been
formed using diamond particles coated with a carbide or carbide
forming material. In contrast, in one aspect of the present
invention, the diamond particles and Cu, Al or Ag can each be used
in their base, untreated form. This can eliminate the costly
process of coating diamond particles prior to infiltrating a mass
of the particles with an infiltrant. In addition, the diamond
particles used in heat spreaders in accordance with the present
invention can utilize diamond particles having a relatively course
grain, for example grains of 50 microns and larger. This can result
in fewer grain boundaries present to slow down heat flow.
[0071] Another way to provide the interstitial material is by
sintering of a solid powder in the voids between carbonaceous
particles. Sintering may be accomplished in a variety of ways,
known to those skilled in the art such as, but not limited to, hot
pressing, pressure-less sintering, vacuum sintering, and microwave
sintering. Although hot pressing is a common method, microwave
sintering is becoming an increasingly useful method as it allows
for faster sintering times and decreased porosity. This is
particularly advantageous in the present invention because the
microwave acts to primarily heat the sinterable metal material
rather than the carbon. When diamond is used, this helps to reduce
degradation of the diamond during processing.
[0072] A sinterable interstitial material may be provided during
the packing process, in which case the sintered material occupies
much of the space between carbonaceous particles and prevents
substantial particle-particle contact. However, the sinterable
interstitial material may be introduced in a similar manner to that
used in successive packing of smaller diamond particles, wherein
the size of the interstitial material is chosen so as to allow the
material to partially fill the voids between carbonaceous particles
after the carbonaceous particles have been packed. Once the voids
are sufficiently filled the interstitial material is sintered. In
this manner the particle-particle contact can be improved.
[0073] A third way to provide the interstitial material is to
infiltrate diamond particles with a molten material (e.g., Al, Si,
BNi2). The electro-deposited metal cannot bond carbon chemically so
carbonaceous particles are entrapped inside. Further, the sintered
material may not hold particles firmly because bonding to
carbonaceous particles during sintering is primarily mechanical.
The infiltrant should contain an active element so it can react
with carbon to form chemical bonds in the form of carbide. The
presence of a carbide former also allows the infiltrant to wet the
particle surface and draw the infiltrant further into the
interstitial voids by capillary action.
[0074] When diamond particles are used, in order to minimize the
diamond degradation, the infiltration is preferably performed at a
temperature below 1,100.degree. C. Many of the Fe, Ni, and Co
alloys mentioned above have melting temperatures in this range.
During infiltration or sintering of an interstitial material, the
hot metal will inevitably cause some small degree of diamond
degradation. However, this effect may be minimized by reducing the
processing time and carefully choosing the interstitial material.
Silicon is particularly good at filling the interstitial voids
between diamond particles due to its tendency to form SiC by
reaction. The formation of SiC at the interface between diamond and
molten Si may protect diamond from further deterioration. The
melting temperature of pure Si is approximately 1,400.degree. C.
Under a high vacuum (e.g. below about 10.sup.-3 such as 10.sup.-5
torr), molten Si or its alloy can infiltrate into diamond
effectively without excessively damaging diamond so a good heat
spreader can be fabricated.
[0075] Thus, the interstitial material may be introduced into the
packed carbonaceous particles by infiltration, sintering or
electro-deposition. When performed at low pressures, these
interstitial materials merely fill the voids between particles and
bond the particles together. At very high pressures there are two
basic possibilities. First, the interstitial material may
chemically bond with the carbon and/or provide beneficial thermal
properties across the carbonaceous material to interstitial
material interface and the carbonaceous material will be partially
crushed to eliminate a portion of the voids. Second, if the
interstitial material is a carbon solvent such as, but not limited
to, iron, cobalt, nickel or alloys of these materials, the
carbonaceous particles will sinter together to form a continuous
carbonaceous mass. When the carbonaceous particles sinter together,
the path for heat transfer is essentially a continuous carbon path
having substantially no mechanical or non-carbon interfaces to
traverse.
[0076] In one embodiment of the present invention, copper is used
as the interstitial material. Copper is an ideal thermal conductor
for making diamond heat spreaders. However, copper is not a carbon
solvent and is not a catalyst for graphite to diamond conversion,
nor does it act as a sintering aid at ultrahigh pressure. Hence, if
copper is used as the interstitial material, it can also be done by
electro-deposition or sintering. However, electro-deposition is
extremely slow and inefficient in filling the pores among tightly
packed diamond grains. Sintering, on the other hand, will
inevitably leave copper caught between diamond grains. In either
method, the carbonaceous particle packing efficiency in the final
heat spreader is relatively low (e.g., 60% by volume).
[0077] Although copper is not a sintering aid to sinter
carbonaceous particles together along carbonaceous grain
boundaries, the ultrahigh pressure consolidation of a carbon-copper
mixture can force carbonaceous grains closer together to reach a
higher carbon content such as 70% by volume. Pressures may range
from about 4 GPa to about 6 GPa. At these high pressures some of
the carbonaceous particles are partially crushed to eliminate a
portion of the voids between particles. In order to attain over 70%
by volume of carbon without forming carbon-to-carbon bridges the
excess copper must be extracted by a sink material. This sink
material contains pores under ultrahigh pressure conditions and
would not soften at the melting temperature of copper. Such a sink
material may be made of a ceramic powder such as SiC, Si3N4, and
Al2O3, but may also be formed of any porous material which provides
a sufficient medium for absorbing the excess copper. Other useful
porous materials include WC and ZrO2. This technique may be further
explained by reference to Example 1 below.
[0078] In another aspect of the present invention, the interstitial
material may be a carbon wetting infiltrating alloy with low
percentage content by volume of the carbon wetting agent. In this
manner, mechanical interfaces between carbon and the infiltrant are
greatly reduced while providing an infiltrant with a relatively
high thermal conductivity. For example, a good heat conducting
metal such as Ag, Cu, or Al, may be alloyed with a carbide former
such as Ti.
[0079] In addition to the benefits provided by including an
infiltrating alloy with good conductivity and diamond wetting
properties, the infiltrating alloy can be selected such that it has
a relatively low eutectic melting point. In this manner the
above-recited disadvantages associated with processing diamond
particles at extremely high temperatures and pressures can be
avoided. In one aspect, the eutectic melting point of the alloy
used may be less than about 1100.degree. C. In another aspect, the
melting point may be less than about 900.degree. C.
[0080] Examples of good carbide forming elements include without
limitation, those recited above. Further, examples of materials
having high heat conductivity include without limitation, Ag, Cu,
and Al. A wide range of specific alloys may be used which attain
the desired heat transfer and chemical bonding properties and also
have a eutectic melting point within the temperatures specified
above. However, in one aspect of the invention, the infiltrating
alloy can include a carbide forming element and at least 1 wt % to
about 10 wt % of either Ag, Cu, or Al. In another aspect, the
carbide forming element is present in an amount of at least about
1% w/w of the heat spreader.
[0081] In another aspect, the infiltrating alloy can comprise a
Cu--Mn alloy. The Cu--Mn alloy can be Cu--Mn(30%)-Ni(5%), which has
a melting point of about 850.degree. C., much lower than sintering
temperatures used in the past. For example, Co, which has a melting
point of about 1500.degree. C., is often used as a sintering aid in
HPHT processes used to form PCD compacts. As previously noted such
temperatures endanger the integrity of the carbonaceous material
and can cause degradation thereof. This is especially true for
diamond particles. In contrast, processing diamond particles at
relatively low temperatures of about 850.degree. C. is much more
desirable, from both an integrity standpoint and a process control
and cost standpoint. In addition to those materials recited above,
the infiltrating alloy used in the present invention can include a
number of other materials. For example, in one aspect of the
invention, CuAlZr(9%)and CuZr(1%) can be used. While Zr is not a
particularly good thermal conductor, its presence in the
infiltrating alloy is relatively small by volume, and therefore
does not significantly inhibit heat conduction through the heat
spreader.
[0082] By utilizing an infiltrating alloy which has a relatively
low melting point and also has good wetting and conductivity
properties, a superior carbonaceous heat spreader can be provided.
The lower operating temperature results in lower operating
pressures. By requiring lower operating pressures, the present
invention can be utilized to form carbonaceous heat spreaders with
far greater thickness than prior art methods, as it has been the
extremely high pressures required to form PCDs that has limited the
mold size used in the past. For instance, in one aspect, the
present invention can be utilized to form heat spreaders with a
thickness greater than about 1 mm. In another aspect, heat
spreaders made in accordance with the present invention can have
thickness as high as and exceeding about 2 mm. By forming the heat
spreader with a greater thickness, the resulting heat spreader has
the capacity to transfer or spread a greater volume of heat per
unit time and therefore has a significantly greater cooling
capacity.
[0083] Heat spreaders made in accordance with the present invention
may take a variety of configurations based on the intended use. The
carbonaceous material made as described above may be polished and
shaped based on the particular requirements of the heat source to
which it will be applied. In contrast to CVD, the carbonaceous
composites herein can be formed to almost any size relatively
quickly. Most often for electronic applications the heat spreader
will be between about 0.1 mm and about 1 mm thick. The heat
spreader may be formed into a circular or elliptical disk or a
quadrilateral such as a square, rectangular or other shaped wafer.
The heat source may be any electrical or other component which
produces heat.
[0084] Once the heat spreader is formed, appropriate placement is
based on design and heat transfer principles. The heat spreader may
be in direct intimate contact with the component, and may even be
formed to encompass or otherwise be contoured to provide direct
contact with the heat source over a wide surface area.
Alternatively, the heat spreader may be removed from the heat
source by a heat conduit or other heat transfer device.
[0085] In addition to the heat spreader disclosed herein, the
present invention encompasses a cooling unit for transferring heat
away from a heat source. As shown in FIG. 1a, a heat spreader 12,
formed in accordance with the principles discussed herein, can be
disposed in thermal communication with both a heat source, such as
a CPU 14, and a heat sink 16. The heat spreader transfers heat
created by the CPU to the heat sink. The heat sink can be a number
of heat sinks known to those of ordinary skill in the art including
both the materials and configurations thereof. For example,
aluminum and copper are well known for use as heat sinks, and as
shown in FIG. 1, can have a configuration that includes cooling
fins 18. As heat is quickly and efficiently transferred from the
CPU through the heat spreader, the heat sink absorbs the heat, and
the cooling fins help dissipate the heat into the surrounding
environment. A number of contact configurations between the heat
sink, heat source, and heat spreader can be utilized depending on
the specific results to be achieved. For example, the components
may be disposed adjacent each other and can also be bonded or
otherwise coupled to each other. In one aspect of the invention,
the heat spreader can be brazed to the heat sink.
[0086] While the heat sink 18 is shown in the figures as a sink
including cooling fins, it is to be understood that the present
invention can be utilized with any heat sink known to those in the
art. Examples of known heat sinks are discussed in U.S. Pat. No.
6,538,892, which is herein incorporated by reference. In one aspect
of the invention, the heat sink comprises a heat pipe having an
internal working fluid. Examples of heat pipe heat sinks are
discussed in U.S. Pat. No. 6,517,221, which is herein incorporated
by reference.
[0087] As shown in FIG. 1b, in one aspect of the invention, the
heat spreader 12 can be at least partially embedded in the heat
sink and/or the heat source. In this manner, not only is heat
transferred from a bottom of the heat spreader to the heat sink,
but heat is also at least partially transferred from sides of the
heat spreader into the heat sink. After being embedded in the heat
sink, the heat spreader can be bonded or brazed to the heat sink.
In one aspect, the heat spreader can be held in the heat sink by a
compression fit. In this manner, no bonding or brazing material
exists between the heat spreader and the heat sink, which might act
as a barrier to efficient heat transfer from the spreader to the
sink.
[0088] While the heat spreader can be held in the heat sink by a
variety of mechanisms known to those skilled in the art, in one
aspect the heat spreader is held in the heat sink by a thermally
induced compression fit. In this embodiment, the heat sink can be
heated to an elevated temperature to expand an opening formed in
the heat sink. The heat spreader can then be fitted into the
expanded opening and the heat sink can be allowed to cool. Upon
cooling, the heat sink, which has a relatively high coefficient of
thermal expansion, will contract around the heat spreader and
create a thermally induced compression fit that holds the heat
spreader embedded within the heat sink without requiring any
intervening bonding material. A mechanical friction fit can also be
utilized to hold the heat spreader in the heat sink.
[0089] As shown in FIG. 1c, in one aspect of the invention, the
heat sink can comprise a heat pipe 22 which can have an internal
working fluid (not shown). The internal working fluid can be any
known to those in the art, and in one aspect is water or water
vapor. The heat pipe can be substantially sealed to maintain the
working fluid within the heat pipe. The heat spreader can be
disposed adjacent the heat pipe and in one aspect is brazed to the
heat pipe. In the embodiment shown in FIG. 1c, the heat spreader
protrudes through a wall of the heat pipe so that a bottom of the
heat spreader is in direct contact with the working fluid. The heat
spreader can be brazed within the heat pipe, as shown at 26, to
assist in maintaining the substantially sealed condition of the
heat pipe.
[0090] As the heat spreader is in direct contact with the working
fluid, the working fluid can more efficiently transfer heat from
the heat spreader. In the embodiment shown in FIG. 1c, the working
fluid, in this case water (not shown), contacts the heat spreader
and becomes vaporized as it absorbs heat from the heat spreader.
The water vapor can then condense in liquid form on the bottom of
the heat pipe, after which, due to capillary forces, the liquid
will migrate 24 back up the walls of the heat pipe to the heat
spreader, where it will again vaporize and repeat the cycle. As the
walls of the heat pipe can be made of a material with a high
coefficient of thermal conductivity, heat is dissipated from the
walls of the heat pipe into the surrounding atmosphere.
[0091] As mentioned above, the packed carbonaceous particles,
especially diamond, may also be sintered together to form a mass of
substantially sintered particles having largely only carbon. When
the carbonaceous particles are sintered together there are carbon
bridges connecting neighboring carbon particles. The
above-described packing methods can increase the original carbon
packing efficiency. By packing different size carbonaceous
particles in successive stages the packing efficiency may be
increased up to about 80% by volume. However, because there is no
carbon-to-carbon bonding, the packing efficiency reaches a limit.
Hence, in order to further increase the packing efficiency and the
thermal conductivity, carbonaceous particles must be sintered
together. In addition, when the carbonaceous particles are sintered
together such that there are carbon bridges connecting neighboring
carbonaceous particles an uninterrupted path for heat flow is
provided. In this way, heat can pass through the carbonaceous heat
spreader rapidly without being slowed down at interfaces between
individual particles which are merely in intimate contact.
[0092] In order for diamond particles to sinter together, they must
be heated in the stability region of diamond, otherwise, diamond
will revert to the more stable form of graphite. U.S. Pat. Nos.
3,574,580; 3,913,280; 4,231,195 and 4,948,388 discuss this process
in more detail and are all incorporated herein by reference.
Diamond sintering is generally performed at very high pressures.
Typically, pressures of more than about 4 GPa up to about 8 GPa are
required, although a few processes have sought to lower this
pressure requirement, e.g., U.S. Pat. No. 4,231,195. More typical
sintering pressure is about 5 to about 6 GPa. At such pressures,
diamond particles sinter together by a mechanism known as liquid
phase sintering.
[0093] An interstitial material may be provided which acts as a
diamond, or carbonaceous particle, sintering aid. During this
process, an interstitial material (e.g., Fe, Co, Ni, Si, Mn, Cr)
can wet the diamond particles. The diamond will dissolve into this
interstitial material because of increased solubility at these
pressures. The local pressure is higher at the contact points of
the diamond particles, so diamond particles will dissolve first at
these points. In contrast, the pressure in the original voids
between diamond particles is low so the dissolved diamond in the
form of carbon atoms in the molten liquid will precipitate out as
diamond in the voids. Hence, the contacting points of diamond will
gradually dissolve and the voids between the diamond particles will
gradually fill with precipitated diamond. The consequence is to
bring diamond particles closer beyond the original contact point
and the substantial elimination of the original voids to produce a
diamond structure having a composition between about 70% and about
98% by volume of diamond. In addition, unlike with the low-pressure
processes described above the diamond particles will not experience
any degradation because the conditions of temperature and pressure
are within the stability region of diamond.
[0094] The final product of ultrahigh pressure sintering of diamond
is a polycrystalline diamond (PCD) with remnant diamond grains
sintered together. In such a structure, the outlines of the
original diamond particles are largely lost and instead prominent
diamond-to-diamond bridges are formed. If diamond sintering can be
performed near completion, the entire mass will be made of diamond
with small pockets of non-diamond material trapped in the original
voids inside the PCD. Such a structure may contain over 95% by
volume of a continuous framework of diamond and hence it is highly
efficient in conducting heat and approaches the thermal properties
of pure diamond.
[0095] This ultrahigh pressure process may also be applied to
carbonaceous composite heat spreaders made by sintering of metal
together at a lower pressure (<2 GPa) as in the case of hot
pressing mentioned above. The ultrahigh pressure process may also
be used to consolidate carbonaceous composite heat spreaders to
increase the carbon content beyond what can be achieved by hot
pressing alone.
[0096] Interstitial materials suitable for the ultrahigh pressure
production of heat spreaders according to the method of the present
invention include Si, Ti, Fe, Co, Ni, Cu, Mn, W, La, Ce, and
mixture or alloys of these materials. Not all of these materials
act as a sintering aid.
[0097] In another aspect of the present invention, a sink material
such as a ceramic is provided to accelerate the removal of the
sintering aids. As described above, this sink material is porous
and does not soften at the ultrahigh pressures used in sintering of
the diamond particles. Such sink materials are most often ceramic
powders such as SiC, Si3N4, and Al.sub.2O.sub.3, but may be any
porous medium which can act to absorb excess sintering aid
material. Other useful porous materials include WC and ZrO2.
[0098] In addition to the above-recited systems and devices, the
present invention also provides a method for making a heat
spreader, and can include the steps of providing a plurality of
carbonaceous particles, and infiltrating the plurality of
carbonaceous particles with a non-carbonaceous infiltrant as
recited in the various aspects described above, such that a heat
conducting mass is formed. In another aspect, a method of cooling a
heat source is provided and includes the steps of providing a heat
spreader as recited in the various aspects described above, and
placing the heat spreader in thermal communication with both the
heat source and a heat sink.
[0099] The following examples present various methods for making
the heat spreaders of the present invention. Such examples are
illustrative only, and no limitation on the present invention is
meant thereby.
EXAMPLES
Example 1
[0100] Diamond particles can be mixed with powdered copper to form
a mixture. This mixture is then cold pressed to form a slug. A thin
walled mold made of a refractory metal (e.g., Ti, Zr, W, Mo, Ta) is
provided. Ceramic particles (e.g., SiC, Si.sub.3N.sub.4,
Al.sub.2O.sub.3) having a coarse grain size (e.g., 40/50 mesh) are
first put in the mold and then the ceramic particles are covered
with the diamond-copper slug. The sample assembly is then placed in
a high-pressure cell and pressurize to over 5GPa. The assembly is
then heat charged to over 1200.degree. C. by passing an electric
current through a heating tube that surrounds the sample assembly.
At this temperature and pressure, copper melts and is forced out
from between the diamond particles. The liquid copper flows to the
bottom of the mold containing the ceramic particles. The ceramic
particles contain ample empty pores to receive the liquid copper.
In this way the diamond grains are partially crushed and
substantially fill in the space left by the copper. The result is a
high diamond content (e.g., 85% by volume) heat spreader. A portion
of the copper remains in the composite material and is bonded to
the diamond to hold the particles together.
[0101] Because of the lack of diamond-to-diamond bridges, the
copper cemented diamond composite described above does not reach a
diamond content of up to 95% by volume of sintered diamond as in
PCD, but its diamond content is much higher than would be produced
by electro-deposition or hot-pressing. Hence, the thermal
conductivity would be much higher than the low-pressure diamond
composite heat spreaders of the present invention. Moreover, the
high thermal conductivity of copper partially compensates for the
lower diamond content (about 80% by volume) when compared to PCD as
the latter contains carbon solvent metals, e.g., Co, that have a
lower thermal conductivity than copper.
[0102] PCD has been made routinely, but is typically designed and
used exclusively for mechanical functions, such as cutting tools,
drill bits, and wire drawing dies. In order to improve the
mechanical finish and to increase the mechanical strength (e.g.,
impact strength), PCD is made of very fine diamond powder. The best
PCD contains very fine diamond particles such as sub-micrometer
sizes (e.g., manufactured by Sumitomo Electric Company of Japan).
By utilizing PCD in a heat spreader, mechanical properties become
less important. Instead of impact strength and surface finish, the
diamond packing efficiency and thermal properties are the primary
concern. Thus, the design of PCD for heat spreaders is distinct
from that of conventional abrasive applications. Specifically, the
diamond particles of the present invention are relatively large
grain sizes, and the infiltrant or sintering aid requires high
thermal conductivity rather than mechanical toughness as in
conventional PCD.
[0103] In order to improve the heat transfer efficiency of the heat
spreader the grain boundaries of diamond particles are minimized,
this is in contrast to a conventional design of diamond composites
where the grain boundaries are maximized. The use of larger diamond
particles not only reduces the grain boundaries that reduce heat
transfer, but also serves to increase the diamond packing
efficiency and further increase the thermal conductivity. Hence,
this design criterion is applicable to all diamond and diamond
composite heat spreaders described herein.
Example 2
[0104] A contact cavity in the form of a circular hole having a
diameter of about 20 mm is formed into a flat base of an aluminum
heat sink with radiating fins cooled by a fan. The heat sink is
heated to a temperature of about 200.degree. C. to expand the
contact cavity, after which a diamond composite heat spreader with
a diameter of about 20 mm is inserted into the contact cavity. Upon
cooling, the much larger thermal expansion contraction of the
aluminum heat sink will result in the diamond composite heat
spreader being firmly compressed into the contact cavity. The top
surface of the diamond composite heat spreader is ground to remove
any debris formed by the shrink fitting. The heat spreader is
placed in contact with a chip or CPU and heat is spread through the
heat spreader and into the heat sink, with heat spreading from both
the bottom of the heat spreader and the sides of the heat spreader
into the heat sink.
Example 3
[0105] 50/60 U.S. mesh diamond particles are acid cleaned and
loaded in a tatalum cup having a cylindrical shape. An oxygen-free
copper disk is placed on top of the diamond particles. The charge
is pressurized to 5.5 GPa in a 2000 ton cubic press that utilizes 6
anvils pressing toward a pyrophllite cube that contained the
charge. Electrical current is passed through a graphite tube that
surrounds the charge. At a temperature of 1150 C, molten copper is
infiltrated through the diamond particles. Upon cooling and
decompression, the charge is ground to remove the tatalium
container and also the top and bottom surfaces of the
diamond-copper composite. The final disk is 37 mm in diameter and 2
mm thick. The diamond content is approximately 82 V %. The
resulting diamond-copper heat spreader has a heat transfer rate of
about 1.5-2 times that of pure copper.
Example 4
[0106] A heat spreader is made in accordance with Example 3, except
that Cu--Zr in concentration of about 1 wt % is used to improve the
wetting characteristics of copper and diamond.
Example 5
[0107] Grafoil is placed in an alumina container and is covered
with 30/40 diamond crystals. The crystals are pressed into the
grafoil by using a flat plate. AgCuSnTi foil is placed on top of
diamond/grafoil mixture. The assembly is heated in a vacuum furnace
at 950 C for 15 minutes. The result is an alloy infiltrated
diamond-graphite.
Example 6
[0108] 30/40 mesh diamond particles (about 500 micrometers) are
mixed with bronze powder (about 20 micrometers) to achieve a volume
efficiency of 50%. The mixture is hot pressed in a graphite mold to
a pressure of 40 MPa (400 atmospheric pressure) and heated to
750.degree. C. for 10 minutes. The result is a diamond metal
composite disk of 30 mm in diameter and 3 mm in thickness.
Example 7
[0109] 30/40 mesh diamond particles are mixed with aluminum powder
and loaded in an alumina tray. The charge is heated in a vacuum
furnace of 10.sup.-5 torr to 700.degree. C. for 5 minutes so the
aluminum becomes molten. After cooling, result is a diamond
aluminum composite.
Example 8
[0110] 30/40 mesh diamond is placed inside a graphite mold and
covered with NICROBRAZ LM (Wall Colmonoy) powder of about 325 mesh.
The load is heated in a vacuum furnace of 10.sup.-5 torr to
1010.degree. C. for 12 minutes. The molten Ni--Cr alloy infiltrated
into diamond particles to form a diamond metal composite.
Example 9
[0111] 30/40 mesh diamond is placed inside a graphite mold and
covered with broken silicon wafers. The load is heated in a vacuum
furnace of 10.sup.-5 torr to 1470.degree. C. for 9 minutes. The
molten Si infiltrated into diamond particles to form a
composite.
Example 10
[0112] 30/40 mesh diamond is placed inside a graphite mold and then
agitated. 220/230 mesh diamond is then placed in the mold and
gently agitated until most of the voids are filled with the smaller
particles. The packed diamond is then covered with NICROBRAZ LM
(Wall Colmonoy) powder of -325 mesh. The load is heated in a vacuum
furnace of 10.sup.-5 torr to 1,010.degree. C. for 12 minutes. The
molten Ni--Cr alloy infiltrated into diamond particles to form a
diamond metal composite.
Example 11
[0113] 30/40 mesh diamond is packed around a cathode and immersed
in an acid bath that contains copper ions. After the current passes
through, copper is gradually deposited in the pores of these
diamond particles. The result is a diamond copper composite.
Example 12
[0114] 20/25 mesh diamond particles (SDA-100S made by De Beers)
substantially cubic in shape were aligned edge to edge on an
alumina plate to form a single layer of diamond particles about 40
mm square. A silicon wafer of 0.7 mm in thickness was placed on top
of this layer of particles. The assembly was then placed in a
vacuum furnace and pumped down to 10.sup.-5 torr. The temperature
was then raised to 1,450.degree. C. for 15 minutes. The silicon
melted and infiltrated between the diamond particles. After
cooling, the composite was machined to eliminate excess silicon.
The result is a diamond heat spreader of about 0.8 mm. This heat
spreader contains a diamond volume of about 90%. The use of
substantially cubic particles allows a much higher diamond content
than can be conventionally achieved using the successive packing
method described earlier.
Example 13
[0115] 40/50 mesh diamond particles are mixed with a mixture of Si
and Ti powders and the entire mixture is loaded inside a graphite
mold that is in turn fitted inside a titanium heating tube. The
assembly is placed at the center of a pyrophyllite block. This
block is mounted in a cubic press and it is subjected to a pressure
of 5.5 GPa. Heating is achieved by passing electrical current
through the titanium tube. When the silicon melts it dissolves
titanium and both flow around the diamond particles. Diamond
particles then sinter with the aid of the silicon liquid. After
quenching and decompression, the diamond composite is separated
from the pyrophillite and other pressure medium. The result is a
diamond composite that contains about 92% by volume of diamond.
Twenty such diamond composites are made each with dimensions of 20
mm in diameter and 3 mm in thickness. These diamond composite disks
were polished by diamond wheels and measured for thermal
conductivity that indicates a value of about twice that of
copper.
[0116] Of course, it is to be understood that the above-described
arrangements are only illustrative of the application of the
principles of the present invention. Numerous modifications and
alternative arrangements may be devised by those skilled in the art
without departing from the spirit and scope of the present
invention and the appended claims are intended to cover such
modifications and arrangements. Thus, while the present invention
has been described above with particularity and detail in
connection with what is presently deemed to be the most practical
and preferred embodiments of the invention, it will be apparent to
those of ordinary skill in the art that numerous modifications,
including, but not limited to, variations in size, materials,
shape, form, function and manner of operation, assembly and use may
be made without departing from the principles and concepts set
forth herein.
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