U.S. patent application number 11/640127 was filed with the patent office on 2007-07-26 for silicon-diamond composite heat spreader and associated methods.
Invention is credited to Chien-Min Sung.
Application Number | 20070170581 11/640127 |
Document ID | / |
Family ID | 32068909 |
Filed Date | 2007-07-26 |
United States Patent
Application |
20070170581 |
Kind Code |
A1 |
Sung; Chien-Min |
July 26, 2007 |
Silicon-diamond composite heat spreader and associated methods
Abstract
Diamond heat spreaders are produced having thermal properties
approaching that of pure diamond. Diamond particles of relatively
large grain size are tightly packed to maximize diamond-to-diamond
contact. Subsequently, smaller diamond particles can optionally be
introduced into the interstitial voids to further increase the
diamond content per volume. An interstitial material which includes
silicon can be used to bond the diamond particles together either
through filling voids between diamond particles or by acting as a
sintering aid. The final heat spreader exhibits superior heat
transfer properties advantageous in removing heat from various
sources such as electronic devices and minimized difference in
thermal expansion from the heat source.
Inventors: |
Sung; Chien-Min; (Taipei
County, TW) |
Correspondence
Address: |
THORPE NORTH & WESTERN, LLP.
8180 SOUTH 700 EAST, SUITE 200
SANDY
UT
84070
US
|
Family ID: |
32068909 |
Appl. No.: |
11/640127 |
Filed: |
December 14, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10270018 |
Oct 11, 2002 |
7173334 |
|
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11640127 |
Dec 14, 2006 |
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Current U.S.
Class: |
257/720 ;
257/E23.11; 257/E23.112 |
Current CPC
Class: |
H01L 23/373 20130101;
H01L 2924/12042 20130101; H01L 23/3732 20130101; H01L 2924/01322
20130101; B22F 2998/00 20130101; H01L 23/3733 20130101; B22F 3/10
20130101; H01L 2924/14 20130101; H01L 2224/8384 20130101; B22F
2998/00 20130101; H01L 2224/29021 20130101; H01L 2924/12042
20130101; H01L 24/29 20130101; H01L 2924/14 20130101; C22C 1/1036
20130101; C22C 26/00 20130101; C22C 2001/1073 20130101; B22F 1/0014
20130101; H01L 2924/00 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
257/720 |
International
Class: |
H01L 23/34 20060101
H01L023/34 |
Claims
1. A diamond composite heat spreader comprising: a) a plurality of
diamond particles, each substantially in intimate diamond-diamond
contact with at least one other diamond particle; and b) an
interstitial material including Si.
2. The heat spreader of claim 1, wherein the plurality of diamond
particles are bound together by sintering under conditions where
the Si acts as a sintering aid.
3. The diamond heat spreader according to claim 2, comprising
between about 70% and about 98% by volume of diamond particles.
4. The heat spreader of claim 1, wherein the interstitial material
substantially binds the plurality of diamond particles in a
composite mass.
5. The heat spreader of either claim 4, wherein the diamond
particles are present in an amount of between about 50% and about
80% by volume of the heat spreader.
6. The heat spreader of claim 1, wherein the diamonds contact each
other sufficiently to provide a continuous diamond-to-diamond path
to substantially each of the diamond particles.
7. The heat spreader of claim 1, wherein the diamond particles have
a substantially single particle size.
8. The heat spreader of claim 1, wherein a first portion of the
diamond particles are of a first average mesh size and a second
portion of the diamond particles are of a second average mesh size
which has a diameter smaller than about 1/3.sup.rd the diameter of
the first mesh size.
9. The heat spreader according to claim 8, wherein the first
portion is at least about 50% of all the diamond particles.
10. The heat spreader of claim 1, wherein the interstitial material
is the result of infiltration or sintering.
11. The heat spreader of claim 1, wherein the interstitial material
is a Si alloy of a member selected from the group consisting of Ni,
Ti, Al, Cr, and combinations thereof.
12. The heat spreader of claim 11, wherein the interstitial
material is a Si alloy of Ti or Al.
13. The heat spreader of claim 1, wherein the interstitial material
consists essentially of silicon.
14. The heat spreader of claim 1, wherein the plurality of diamond
particles are randomly packed.
15. The heat spreader of claim 1, wherein the diamond composite
heat spreader has a thermal conductivity about two times that of
copper or greater.
16. The heat spreader of claim 1, further comprising a heat source
attached to the heat spreader.
17. A method of making a diamond composite heat spreader comprising
the steps of: a) providing a first plurality of diamond particles
having a first average mesh size; b) packing the diamond particles
such that substantially each diamond particle is substantially in
intimate diamond-diamond contact with at least one other diamond
particle; c) providing an interstitial material including Si; and
d) bonding the packed diamond particles using the interstitial
material.
18. The method of claim 17, wherein the interstitial material at
least partially fills any voids between the packed diamond
particles and acts to acts to substantially bind the plurality of
diamond particles into a composite mass.
19. The method of claim 18, wherein the step of bonding is
performed by sintering or infiltration of the interstitial
material.
20. The method of claim 19, wherein infiltration or sintering is
performed at a temperature below about 1,100.degree. C.
21. The method of claim 19, wherein infiltration is performed in a
vacuum furnace at a pressure below about 10.sup.-3 torr.
22. The method of claim 17, wherein the step of bonding includes
sintering the plurality of diamond particles in the presence of the
interstitial material such that the diamond particles partially
sinter together to provide a substantially sintered mass of diamond
particles.
23. The method of claim 17, wherein the interstitial material is a
Si alloy of a member selected from the group consisting of Ni, Ti,
Al, Cr, and combinations thereof.
24. The method of claim 23, wherein the interstitial material is a
Si alloy of Ti or Al.
25. The method of claim 17, wherein the interstitial material
consists essentially of silicon.
26. The method of claim 17, wherein the step of packing further
comprises packing diamonds to over 50% by volume of the heat
spreader prior to providing an interstitial material.
27. The method of claim 17, wherein prior to the step of providing
an interstitial material, the method further comprises the step of
adding a second plurality of diamond particles having a second
average mesh size smaller than the first mesh size to the packed
diamond particles such that the second plurality of diamond
particles at least partially fill in the voids between the larger
particles to produce a packed collection of diamond between about
50% and about 80% by volume of diamond.
28. The method of claim 27, wherein the second mesh size particles
have a diameter of between about 1/5.sup.th and about 1/10.sup.th
the diameter of the first average mesh size particles.
29. The method of claim 17, wherein the diamond particles contact
one another sufficiently to provide a continuous diamond-to-diamond
path to substantially each of the plurality of diamond
particles.
30. The method of making a diamond composite heat spreader
according to claim 17, further comprising the steps of: a)
providing a porous ceramic material prior to the step of bonding;
and b) placing the ceramic material adjacent to the packed diamond
particles prior to the step of bonding.
31. The method of making a diamond composite heat spreader
according to claim 30, wherein the ceramic material comprises at
least 50% by volume of a member selected from the group consisting
of SiC, Si.sub.3N.sub.4, Al.sub.2O.sub.3, WC, and ZrO.sub.2.
32. A method of removing heat from a heat source comprising the
steps of: a) providing a heat spreader as recited in claim 1; and
b) positioning the heat spreader in operative connection with the
heat source.
33. The method of claim 32, wherein the heat source is a CPU.
Description
CLAIM OF PRIORITY
[0001] This application is a continuation application of U.S.
patent application Ser. No. 10/270,018, filed Oct. 11, 2002, which
is hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to diamond composite devices
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. The state-of-the-art CPU in
2002 has a power of about 60 watts (W). CPUs made with 0.13
micrometer technology will 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
conductor (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 of 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] The above heat dissipation methods are only effective if the
power of the CPU is less than about 60 W. For CPUs with higher
power, more effective means must be sought to keep the hot spot of
the chip below the temperature threshold.
[0009] 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.
[0010] 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.
[0011] One promising alternative that has been explored for use in
heat sinks 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 I 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) (l/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 Silicon 148 1.66 2.58E-6 Diamond
(IIa) 2,300 1.78 1.4E-6
[0012] 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.
[0013] In recent years diamond heat spreaders have been used to
dissipate heat from high power laser diodes, such as that used 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.
[0014] Many current diamond heat spreaders are made of diamond
films formed by chemical vapor deposition (CVD). The raw CVD
diamond films are now sold at over $10/cm.sup.2, and this price may
double 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.
[0015] As such, cost effective 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 diamond
composite heat spreaders that can be used to draw or conduct heat
away from a heat source. In one aspect, the diamond composite heat
spreader may include a plurality of diamond particles which are
each substantially in contact with another particle and an
interstitial material which binds the diamond particles into a
composite mass.
[0017] In a more detailed aspect of the present invention, each of
the diamonds are in such intimate contact that there is a
continuous diamond-to-diamond path to each of the diamond
particles. This can be generally accomplished by first packing the
diamond particles in the absence of non-diamond material. In yet a
more detailed aspect, the volume content of diamond may be
increased by providing a portion of diamond particles which are
smaller than the first packed diamond particles. In this way, the
smaller diamonds partially fill the voids between the larger
particles. Several successive packing steps using various size
diamonds may be performed. The resulting percent by volume of
diamond in the composite may range from about 50% to about 80%
using this method.
[0018] In order to bind the particles together into a composite
mass, an interstitial material may be introduced which meets
certain thermal properties, such as thermal conductivity, thermal
capacity and thermal expansion. The interstitial material may be
introduced via infiltration, sintering, or electro-deposition.
[0019] In one aspect of the present invention, the interstitial
material may contain an element such as Ag, Cu, Al, Si, Fe, Ni, Co,
Mn, W, and alloys or mixtures of these elements. The presence of a
carbide former in the interstitial material may aid in producing a
composite having intimate contact with the diamond particles and
increasing thermal conductivity throughout the diamond
composite.
[0020] In a still more detailed aspect of the present invention, a
metal or ceramic interstitial material is introduced into the
packed diamond particles. A porous ceramic material is then placed
adjacent to the diamond particles prior to bonding the particles
together. The diamond particles and ceramic material is then
subjected to ultrahigh pressures between about 4 GPa (gigapascal)
and about 6 GPa, and heated, typically by passing an electrical
current through a conductor. Under these conditions, a portion of
the interstitial material flows from the packed diamond into the
porous ceramic, while the remaining interstitial material bonds the
diamond particles together into a composite mass.
[0021] In yet another aspect of the present invention, the packed
diamond particles may be sintered at an ultrahigh pressure in the
presence of an interstitial material which aids in the sintering
process. These interstitial materials may contain an element such
as Si, Ti, Ni, Fe, Co, Cu, Mn, Cr, La, Ce, or their alloys or
mixtures. Pressures of between about 4 GPa and 8 GPa and
temperatures between about 1,000.degree. C. and about 2,000.degree.
C. are applied in order to achieve substantial sintering of the
diamond particles. The resulting diamond heat spreader may contain
between about 70% and about 98% by volume of diamond. By sintering
diamond particles together, compositions above about 90% by volume
of diamond can be achieved to produce a diamond heat spreader
having thermal properties approaching that of pure diamond at a
fraction of the cost.
[0022] Heat spreaders of the present invention may be positioned at
or near a heat source such that the heat spreader effectively
conducts or carries the heat away from the heat source. Thus the
present invention provides a cost effective heat spreader for use
in connection with a heat source, such as semiconductor chips. In
another aspect, the present invention provides a method to make
heat spreaders of a variety of thicknesses and shapes which are
suitable for high power consumption electronic components such as
CPUs. In yet another aspect the present invention to provides a
composite heat spreader having a thermal expansion coefficient
which can be adjusted to match the heat source to which it is
attached.
[0023] 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.
DETAILED DESCRIPTION
[0024] 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.
[0025] 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.
Definitions
[0026] In describing and claiming the present invention, the
following terminology will be used in accordance with the
definitions set forth below.
[0027] As used herein, "particle" and "grit" may be used
interchangeably, and when used in connection with a diamond
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.
[0028] 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, diamond 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, diamond 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.
[0029] 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 heat
spreaders do not retain a significant amount of heat, but merely
transfer heat away from a heat source.
[0030] 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
a diamond 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.
[0031] As used herein "wetting" refers to the process of flowing a
molten metal across at least a portion of the surface of a
superabrasive particle. Wetting is often due, at least in part to
the surface tension of the molten metal, and leads to the forming
chemical bonds between the superabrasive particle and the molten
metal at the interface thereof. Accordingly, a tool having
superabrasive particles that are "wet" by a metal indicates the
existence of chemical bonds between the superabrasive particles and
the metal at the interface thereof.
[0032] As used herein, "chemical bond" and "chemical bonding" may
be used interchangeably, and refer to a molecular bond that exert
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.
[0033] 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 "brazed"
may be used to refer to the formation of chemical bonds between a
superabrasive particle and a braze alloy.
[0034] 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 a metal or 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.
[0035] 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.
[0036] 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.
[0037] 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.
The Invention
[0038] The present invention encompasses devices and methods for
transferring heat away from a heat source. Heat spreaders made in
accordance with the method of the present invention contain a
plurality of diamond particles, each substantially in contact with
one another. The plurality of diamond particles may be bound
together using an interstitial material or by sintering of the
diamond particles.
[0039] In either case, a first plurality of diamond particles are
packed in a suitable mold. The first plurality of diamond particles
are each approximately the same mesh size. The specific size of
these particles is up to about 18 mesh (1 mm) with sizes between
about 30 mesh (0.5 mm) and about 400 mesh (37 micrometers) being
typical. The size of these diamond particles may vary but it is
recognized that larger diamond particles provide for a larger path
having the desirable heat transfer characteristics which approach
that of pure diamond. These diamond particles are packed such that
there is substantial diamond-diamond 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 diamond particles may be sufficient to provide a continuous
diamond-to-diamond path to substantially all of the diamond
particles in the heat spreader. The transfer of heat away from the
heat source is facilitated when there is substantial
diamond-diamond contact, as opposed to empty voids or other
non-diamond material. The diamond particles are packed so as to
occupy most of the heat spreader volume and minimize the amount of
empty void between particles.
[0040] An interstitial material may be used in connection with the
diamond particles in order to bond the diamond particles together
into a composite mass as is discussed in more detail below. By
packing the particles prior to introduction of the interstitial
material, the original diamond-to-diamond contact can be maintained
so the packing efficiency may exceed one-half. This method can
achieve a diamond volume packing efficiency of up to two-thirds. In
contrast, if the diamond particles are mixed with an interstitial
material and then sintered by hot pressing, the consolidated mass
is likely to contain less than one-half of the volume as diamond.
This is because interstitial material tends to fill around diamond
particles and between them. In this case, diamond particles are
separated by the consolidated interstitial material and heat must
cross significant areas of non-diamond material.
[0041] In an additional aspect of the present invention, even
higher packing efficiency may be achieved by packing diamond
particles of different sizes in successive stages. For example, a
larger diamond is packed into a suitable mold. The packing of the
diamond particles may be improved by settling or otherwise
compacting, e.g., agitated inside the mold by a vibrator. A
plurality of smaller diamond particles may then be added to fill
the voids surrounding the larger diamond particles. Depending on
the size of the smaller diamond, the smaller diamond may need to be
introduced from multiple sides of the packed diamond in order to
fill most of the available voids. The size of the smaller diamond
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 two-thirds. If
necessary, addition of even smaller diamond 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. Ultimately,
the packing efficiency will asymptotically approach a level of
about three-fourths. The packed diamond particles made in
accordance with the above principles in mind will provide a diamond
volume content of between about 50% and about 80%.
[0042] In an alternative embodiment, the different size diamond
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 diamonds not being in
intimate contact with other larger particles. Thus, heat must cross
a greater number of diamond-diamond interface boundaries increasing
the thermal resistance of the final heat spreader.
[0043] In yet another alternative embodiment, the volume of diamond
is increased by using uniformly shaped diamond 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%.
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.
[0044] The present invention encompasses a diamond composite heat
spreader which takes advantage of the unique thermal properties of
diamond and reducing the manufacturing cost of such heat spreaders.
Diamond synthesized under ultrahigh pressure can grow more than
1000 micrometers per hour and is much faster than 1-10 micrometers
per hour for typical CVD processes. Consequently, the cost of
high-pressure synthesized diamond is significantly lower ($0.05 per
carat) than the cost for CVD diamond (above $4 per carat). However,
unlike CVD diamond that is grown as a film, high-pressure diamond
is provided in particulate form. It would be desirable to bond
diamond particles together to form a continual network of diamond.
However, in addition to being the hardest material known, diamond
is also the most inert. Hence, there are currently no easy ways to
chemically bond diamond particles together.
[0045] Thus, in accordance with one aspect of the present
invention, the diamond particles are packed before the introduction
of any non-diamond materials as discussed above. One factor to
consider in designing a diamond composite heat spreader of the
present invention is the thermal properties of the composite at the
interfaces between diamond particles and the interfaces between
non-diamond material and diamond particles. Empty voids and mere
mechanical contact between diamond-diamond or non-diamond-diamond
interfaces acts as a thermal barrier. Although intimate contact of
diamond particles along a significant portion of the surface of
diamond particles improves the thermal properties at these
boundaries, the result is somewhat inferior to that of pure
continuous diamond. Thus, it is desirable that a substantial
portion of the interfaces are more than mere mechanical
contact.
[0046] Accordingly, an interstitial material is provided with the
choice of any particular interstitial material depending on the
manner in which the particles are to be bound together. In one
aspect of the present invention, the packed diamond particles are
bound by the interstitial material, preferably by forming chemical
bonds. In another aspect of the present invention, the interstitial
material acts as a diamond sintering aid under ultrahigh pressure
to sinter the diamond particles together.
[0047] The choice of interstitial material must account for the
thermal conductivity and 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.
[0048] The interstitial material for bonding or sintering of
diamond 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 in solution on the surface of the diamond particles under an
electrical current.
[0049] Two basic categories of interstitial material include liquid
metal and molten ceramics. When bonding the diamond particles to
produce a diamond composite heat spreader the interstitial material
should contain at least one active element that will react with
diamond 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 diamond particles to produce a diamond
heat spreader, the interstitial material should act as a sintering
aid to increase the degree of diamond sintering and does not
necessarily contain a carbide former but rather containing a carbon
solvent.
[0050] An additional consideration in choosing an interstitial
material is that the infiltration or sintering temperature of the
interstitial material may not be so high as 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.
[0051] In accordance with the present invention, interstitial
materials may contain a diamond braze as a metal infiltrant or
silicon alloys as ceramic infiltrants. Moreover, the infiltrant
must be able to "wet" diamond so it can be wicked in the
interstitial of diamond particles by capillary force. The
interstitial material substantially fills any of the remaining
voids between the packed diamond particles. Common diamond wetting
agents include Co, Ni, Fe, Si, Mn, and Cr. When the diamond
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 Ti, V, Cr, Zr, Mo, W, Mn, Si, Fe, and
Al.
[0052] Interstitial 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 brazes include Fe, Co, or Ni alloys
which exhibit wetting of the diamond 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 BNi.sub.2 (Ni--Cr--B) or BNi.sub.7
(Ni--Cr--P) are good diamond infiltrants. Other examples of
effective infiltrants include Al--Si, Cu--Sn--Ti, Ag--Cu--Ti, and
Cu--Ni--Zr--Ti. Most diamond interstitial materials contain active
elements (e.g., Cr, Ti) that not only bond to diamond 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.
[0053] The above diamond composite heat spreaders can be produced
by at least partially filling in the pores or gaps among diamond
particles by an interstitial material that can conduct heat
relatively fast. The interstitial material may be introduced into
the packed diamonds in a variety of ways. One way to provide the
interstitial material is by electro-deposition (e.g., Ag, Cu, and
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.
[0054] Another way to provide the interstitial material is by
sintering of a solid powder in the voids between diamond 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,
pressureless 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
diamond. This helps to reduce degradation of the diamond during
processing.
[0055] A sinterable interstitial material may be provided during
the packing process, in which case the sintered material occupies
much of the space between diamond particles and prevents
substantial diamond-diamond 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 diamond particles
after the diamond particles have been packed. Once the voids are
sufficiently filled the interstitial material is sintered. In this
manner the diamond-diamond contact can be improved.
[0056] A third way to provide the interstitial material is to
infiltrate diamond particles with a molten material (e.g., Al, Si,
and BNi.sub.2). The electro-deposited metal cannot bond diamond
chemically so diamond particles are entrapped inside. Further, the
sintered material may not hold diamond firmly because bonding to
diamond during sintering is primarily mechanical. The infiltrant
should contain an active element so it can react with diamond to
form chemical bonds in the form of a carbide. The presence of a
carbide former also allows the infiltrant to wet the diamond
surface and draw the infiltrant further into the interstitial voids
by capillary action.
[0057] 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.
[0058] Thus, the interstitial material may be introduced into the
packed diamond by infiltration, sintering or electro-deposition.
When performed at low pressures, these interstitial materials
merely fill the voids between diamond particles and bond the
particles together. At very high pressures there are two basic
possibilities. First, the interstitial material may chemically bond
with the diamond and/or provide beneficial thermal properties
across the diamond to interstitial material to diamond interface
and the diamond 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 diamond particles will sinter together to form
a continuous diamond mass. When the diamond particles sinter
together, the path for heat transfer is essentially a continuous
diamond path having substantially no mechanical or non-diamond
interfaces to traverse.
[0059] 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 diamond packing efficiency in the final diamond heat
spreader is relatively low (e.g., 60% by volume).
[0060] Although copper is not a sintering aid to sinter diamond
particles together along diamond grain boundaries, the ultrahigh
pressure consolidation of a diamond-copper mixture can force
diamond grains closer together to reach a higher diamond content
such as 70% by volume. Pressures may range from about 4 GPa to
about 6 GPa. At these high pressures some of the diamond particles
are partially crushed to eliminate a portion of the voids between
particles. In order to attain over 70% by volume of diamond without
forming diamond-to-diamond 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, Si.sub.3N.sub.4, and Al.sub.2O.sub.3,
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 ZrO.sub.2. This technique may be
further explained by reference to Example 1 below.
[0061] Heat spreaders made in accordance with the present invention
may take a variety of configurations based on the intended use. The
diamond 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 diamond composites herein
can be formed to almost any size relatively quickly. Most often for
electronic applications the heat spreader will be between about 1
mm and about 5 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.
[0062] 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.
[0063] As mentioned above, the packed diamond particles may also be
sintered together to form a mass of substantially sintered
particles having largely only diamond. When the diamond particles
are sintered together there are diamond bridges connecting
neighboring diamond particles. The above-described packing methods
can increase the original diamond packing efficiency. By packing
different size diamond particles in successive stages the packing
efficiency may be increased up to about 80% by volume. However,
because there is no diamond-to-diamond bonding, the packing
efficiency reaches a limit. Hence, in order to further increase the
packing efficiency and the thermal conductivity, diamond particles
must be sintered together. In addition, when the diamond particles
are sintered together such that there are diamond bridges
connecting neighboring diamond particles an uninterrupted path for
heat flow is provided. In this way, heat can pass through the
diamond heat spreader rapidly without being slowed down at
interfaces between individual particles which are merely in
intimate contact. The presence of a carbide forming interstitial
material helps to improve the thermal conductivity, however such a
bond is inferior in thermal properties to that of pure diamond or
sintered diamond.
[0064] 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.
[0065] An interstitial material may be provided which acts as a
diamond sintering aid. During this process, an interstitial
material (e.g., Fe, Co, Ni, Si, Mn, and 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.
[0066] 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.
[0067] This ultrahigh pressure process may also be applied to
diamond 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 diamond composite heat spreaders to increase
the diamond content beyond what can be achieved by hot pressing
alone.
[0068] 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.
[0069] There are several considerations in producing diamond heat
spreaders in accordance with the present invention. The goal is to
take advantage of the unique thermal properties of diamond while
also keeping manufacturing costs to a minimum. A high diamond
volume percent in combination with using large grain diamond
particles minimizes the non-diamond thermal characteristics of the
final heat spreader. Further, interstitial materials having a high
thermal conductivity are desirable. Intimate contact of diamond and
non-diamond material at their interface reduces the volume of poor
heat conducting voids. The formation of carbide bonds further
enhances the heat transfer across these interfaces.
[0070] High diamond volume occupancy and large grain-size are
contrary to conventional wisdom in designing such composite
materials. Non-diamond material having high thermal conductivity
and intimate diamond-diamond contact are contradictory to each
other. Specifically Ag and Cu have high thermal conductivity, but
they do not react with diamond, so the interface is a mechanical
joint that may become a thermal barrier. On the other hand, Si and
diamond braze (e.g., BNi.sub.2 or BNi.sub.7) can wet and form
chemical bonds with diamond, but such materials conduct heat less
effectively than Ag or Cu. Thus, using Ag or Cu in combination with
a carbide former produces a product with improved thermal
properties over that of composites using each alone. Not all
carbide formers produce a useful product, however. For example,
aluminum can react with diamond to produce a composite which
conducts heat relatively fast. However, aluminum carbide located at
the interface is relatively unstable, in particular with respect to
hydrolysis, i.e., it may react with moisture in air. As for carbon
solvents, such as Fe, Co, Ni, Mn, La, Ce, their thermal
conductivity is relative poor so they are only moderately
advantageous in making the diamond composite material at low
pressures. In particular, these carbon solvents are also the
catalyst for synthesizing diamond under ultrahigh pressure, i.e.,
they aid in the conversion of graphite to diamond in the stability
region of diamond. However, in the stability region of graphite,
these carbon solvents will also reduce diamond back to carbon at a
temperature above 700.degree. C. Hence, by sintering such elements
(e.g., by hot pressing) at a high temperature, diamond may
deteriorate and lose its superior thermal properties. However, at
ultrahigh pressures these carbon solvents can aid in sintering the
diamond grains by forming diamond-to-diamond bridges. The result is
an efficient heat spreader with very high diamond content (e.g.,
more than 90% by volume).
[0071] 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, Si.sub.3N.sub.4, 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
ZrO.sub.2.
[0072] 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
[0073] 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, and
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 5 GPa. 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.
[0074] 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.
[0075] 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.
[0076] 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
[0077] 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 3
[0078] 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 4
[0079] 30/40 mesh diamond is placed inside a graphite mold and
covered with NICROBRAZ LM (Wall Colmonoy) powder of -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 5
[0080] 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 6
[0081] 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 7
[0082] 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 8
[0083] 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 9
[0084] 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 pyrophyllite 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.
[0085] 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.
* * * * *