U.S. patent application number 09/981952 was filed with the patent office on 2002-02-28 for high-pressure high-temperature polycrystalline diamond heat spreader.
Invention is credited to Fox, Joe R., Hall, David R..
Application Number | 20020023733 09/981952 |
Document ID | / |
Family ID | 23827402 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020023733 |
Kind Code |
A1 |
Hall, David R. ; et
al. |
February 28, 2002 |
High-pressure high-temperature polycrystalline diamond heat
spreader
Abstract
This invention presents a polycrystalline diamond heat spreader
useful in electronic devices for transmitting thermal energy from a
high-energy thermal source, such as an IC die, into a means for
dissipating thermal energy, such as a heat sink. The heat spreader
comprises a bondable material that forms in situ at high pressure
and high temperature a low-impedance contact surface layer on at
least one its major surfaces. The contact surface layer provides a
means for chemically or metallurgically bonding the heat spreader
to the IC die and or to the heat sink.
Inventors: |
Hall, David R.; (Provo,
UT) ; Fox, Joe R.; (Provo, UT) |
Correspondence
Address: |
David R. Hall
2185 S. Larsen Parkway
Provo
UT
84606
US
|
Family ID: |
23827402 |
Appl. No.: |
09/981952 |
Filed: |
October 18, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09981952 |
Oct 18, 2001 |
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09460105 |
Dec 13, 1999 |
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Current U.S.
Class: |
165/80.3 ;
165/185; 257/E23.098; 257/E23.111 |
Current CPC
Class: |
H01L 2924/00 20130101;
H01L 2924/0002 20130101; H01L 2924/3011 20130101; F28F 2255/18
20130101; H01L 23/3732 20130101; H01L 2924/0002 20130101; F28F
13/00 20130101; F28F 2013/005 20130101; H01L 23/473 20130101 |
Class at
Publication: |
165/80.3 ;
165/185 |
International
Class: |
F28F 007/00 |
Claims
What is claimed:
1. A high-pressure high-temperature polycrystalline diamond heat
spreader useful in electronic devices, comprising: a. Discrete
diamond crystals ranging in sizes from between 10.mu. and 3000.mu.
intergrown into a unified matrix and bonded to a substrate by means
of a high-pressure high-temperature sintering process; b. The
unified matrix having a major surface opposite the substrate; c.
The unified matrix further comprising at least one bondable
material selected from the periodic table of elements consisting of
Fe, Co, Ni, Pd, Pt, Cr, Mo, W, Nb, Ta, Hf, Zr, Ti, V, Al, Si, Ga,
Au, Cd, Ag, An, Mg, Sn, and Ge, and compounds, carbides, and alloys
thereof; and d. The bondable material forming in situ, during the
sintering process, a low-impedance, bondable contact-surface layer
on at least a portion of the major surface opposite the substrate,
said bondable surface being suitable for bonding said matrix to at
least a portion of a high-energy thermal source.
2. The heat spreader of claim 1, wherein the bondable material does
not exceed the weight of the diamond matrix.
3. The heat spreader of claim 1, wherein the bondable material does
not exceed 20 weight-percent of the diamond matrix.
4. The heat spreader of claim 1, wherein the bondable material is
in the form of a foil positioned adjacent the diamond crystals
prior to being subjected to the high-pressure high-temperature
sintering process.
5. The heat spreader of claim 1, wherein the bondable material is
in the form of a powder positioned adjacent the diamond crystals
prior to being subjected to the high-pressure high-temperature
sintering process.
6. The heat spreader of claim 15, wherein the bondable material is
intermixed with the discrete diamond crystals prior to being
intergrown by the high-pressure high-temperature sintering
process.
7. The heat spreader of claim 1, wherein the diamond crystals are
coated with the bondable material prior to being subjected to the
high-pressure high-temperature sintering process.
8. The heat spreader of claim 1, wherein the diamond crystals are
sputter coated with the bondable material prior to being subject to
the high-pressure high-temperature sintering process.
9. The heat spreader of claim 1, wherein the diamond crystals are
coated with the bondable material using an ion implantation process
prior to the sintering process.
10. The heat spreader of claim 1, wherein the diamond crystals are
coated with the bondable material using a milling process.
11. The heat spreader of claim 1, wherein the substrate is adapted
for bonding to a heat sink.
12. The heat spreader of claim 1, wherein the substrate is adapted
for bonding to a thermal source.
13. The heat spreader of claim 1, wherein at least a portion of at
least one major contact surface has low surface roughness.
14. The heat spreader of claim 1, wherein at least a portion of at
least one major contact surface has a non-planar topography.
15. The heat spreader of claim 1, wherein at least a portion of the
bondable surface has a non-planar topography.
16. The heat spreader of claim 1, wherein the bondable surface
layer has a cross section of 1.5 mm, but preferably less than 0.25
mm, and more preferably less than 0.025 mm.
17. The heat spreader of claim 1, wherein at least a portion of one
of the major contact surfaces are made to have high electrical
resistance.
18. The heat spreader of claim 1, wherein the diamond matrix has a
cross section greater than 1 mm.
Description
RELATED APPLICATIONS
[0001] Continuation in Part of U.S. application Ser. No.
09/460,105
Background of the Invention
[0002] This invention relates to a high-pressure high-temperature
polycrystalline diamond material having high thermal conductivity
useful in electronic devices for spreading the thermal energy
produced by a high-density thermal source, such as an IC die, into
a convective means for dissipating heat, such as a heat sink. More
particularly, this invention relates to a polycrystalline diamond
heat spreader that has a low-impedance layer of bondable material
on at least one of its major surfaces that is economically formed
in situ during the high pressure and high temperature process. The
bondable layer on the heat spreader provides a means for intimately
coupling with the IC die and or the heat sink.
[0003] The continued development of high-speed electronic devices,
such as IC dies, produces increasing heat densities in such devices
that are destructive to the device, itself. Currently these
densities exceed 200,000 w/m.sup.2, or roughly the equivalent of
the heat density on the sun's surface. Thermal management, or the
dissipation of the thermal energy in such devices, has become a
major technological hurdle for manufacturers; in fact, the
efficient dissipation of thermal energy is more and more becoming a
major impediment to the further development of such devices.
[0004] The non-abrasive and thermal properties of a high-pressure
high-temperature polycrystalline diamond make it an ideal material
for use in a thermal management system for dissipating thermal
energy.
[0005] First of all, high-pressure high-temperature polycrystalline
diamond is the most economical form of man-made diamond, costing
less than {fraction (1/20)}.sup.th the cost of any other form of
diamond material. Second, it is a mature product whose production
has been known in the art for nearly 50 years. It is produced from
natural or synthetic discrete diamond crystals that may vary in
size and are typically mounted onto metal carbide subtrate. A known
catalyst material of up to approximately 30% by volume is provided
by either mixing it with the diamond or by infiltration from the
metal binder used in the carbide substrate. The catalyst may also
be provided by a combination of mixing and infiltration. This
assembly is sealed inside a refractory metal can and loaded inside
a reaction vessel well known in the art. The reaction vessel is
inserted into a high-pressure high-temperature press apparatus also
known in the art and taken to pressure and temperature regions
where diamond is thermodynamically stable. Under these conditions
the catalyst melts and infiltrates, or sweeps through, the diamond
mass, resulting in diamond-to-diamond bonding, or intergrowth. The
result is a matrix of polycrystalline diamond material that
possesses the combined properties of its catalyst material and its
natural or synthetic diamond constituents: strong, rigid, high in
thermal conductivity, low in thermal capacity, black in color, and
it has a coefficient of thermal expansion compatible with that of
silicon.
[0006] Next, high-pressure high-temperature polycrystalline diamond
may be manufactured into a variety of configurations by either
performing the substrate or by using standard machine shop art. For
example, by varying the interfacial surface of the metal carbide
substrate onto which the discrete diamond crystals are mounted, the
shape and surface topography of the diamond matrix may take on
planar and non-planar shapes. These shapes may be further modified
to achieve multiple configurations and surfaces having low surface
roughness, which adds to the versatility of this form of
polycrystalline diamond. Furthermore, polycrystalline diamond may
be made electrically resistive by either chemically or mechanically
removing its residual catalytic content.
[0007] The objective of a thermal management system is to take the
heat away from the thermal source as quickly as possible and
deliver it to a convective means for dissipation into the
atmosphere. An obstacle common in any thermal management system is
the thermal resistance of the materials being used and the contact
surface, or junction, thermal resistance arising where such
materials are joined. High-pressure high-temperature
polycrystalline diamond has the lowest thermal resistance, or
highest thermal conductivity, of any material, except in some cases
diamond itself. However, in the chain of materials used to transmit
heat from the thermal source and ultimately into the atmosphere,
even where polycrystalline diamond is used, it has been found that
thermal resistance at the junctions may be higher than the thermal
resistance of all the other elements in the thermal chain. Thus
contact surface resistance must be reduced in order to take full
advantage diamond's high thermal conductivity in a thermal
management system.
[0008] The reduction of contact surface resistance may be achieved
by intimately coupling the polycrystalline diamond heat spreader to
the thermal source and or to the heat sink. However, in the past,
intimately coupling with diamond has been an expensive and
difficult obstacle since standard bonding materials are reluctant
to wet diamond due to its chemical inertness. This obstacle is
overcome by this invention by providing a polycrystalline diamond
material that has a bondable contact surface of low thermal
impedance, economically formed in situ at high pressure and high
temperature on at least one surface of the polycrystalline diamond
heat spreader.
SUMMARY OF THE INVENTION
[0009] This invention presents a polycrystalline diamond heat
spreader for use in electronic devices having low thermal impedance
and bondable contact surfaces that are formed in situ during the
high-pressure high-temperature process. Polycrystalline diamond of
this invention is produced by the high-pressure high-temperature
sintering method from discrete natural or synthetic diamond
crystals ranging in sizes from between 10.mu. to 3000.mu. that are
intergrown into a unified matrix. Because polycrystalline diamond
is produced from discrete crystals that are mounted on a substrate,
by varying the shape of the substrate's interfacial surface, the
polycrystalline diamond matrix may be pre-configured having a
variety of planar and non-planar surface topographies. Furthermore,
the polycrystalline diamond of this invention is black in color,
strong and rigid, and may be made having high electrical
resistance, low surface roughness, high thermal conductivity, low
thermal capacity, and a coefficient of thermal expansion compatible
with silicon. Therefore, it is well suited to be bonded adjacent to
the silicon IC as well as to the heat sink. Moreover, the
inter-crystalline intergrowth of polycrystalline diamond provides
an uninterrupted, low-impedance thermal path for transmitting heat
between the heat source and the sink.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a perspective diagram of a heat spreader of the
present invention having a bondable layer on at least one major
surface.
[0011] FIG. 2 is a sectioned representation of a non-planar heat
spreader having a convex profile.
[0012] FIG. 3 is a sectioned representation of a non-planar heat
spreader having an interrupted surface profile.
[0013] FIG. 4 is a diagram of polycrystalline diamond intergrown by
the high-pressure high-temperature sintering method depicting
thermal paths through the matrix.
[0014] FIG. 5 is a cross-section diagram of the polycrystalline
diamond heat spreader positioned between an electronic heat source
and the means for dissipating thermal energy.
DETAILED DESCRIPTION OF THE INVENTION
[0015] This invention presents a high-pressure high-temperature
polycrystalline diamond heat spreader for use in electronic devices
having high thermal conductivity and comprising a bondable contact
surface opposite the substrate having low thermal impedance that is
economically formed in situ during the high-pressure
high-temperature sintering process. The bondable contact surface
permits bonding of the heat spreader to a thermal source or to a
heat sink. The contact surface may also be made to be planar or
non-planar, to have low surface roughness, and to have high
electrical resistance.
[0016] Thermal energy is transmitted in diamond faster than in any
other material at ambient temperatures. Heat is transmitted through
a metal by electron transport and through a crystal, such as
diamond, by phonon, or acoustic, transport. In either case there is
some thermal resistance. Also, where materials are joined in a
thermal chain, intimate coupling of materials is necessary in order
to reduce junction thermal resistance and maximize thermal energy
transmission. Studies have shown that thermal resistance is highest
at the contact surfaces, or junctions, where materials are joined
together, making contact surface resistance as significant a
consideration in the thermal chain as the actual thermal
conductivity of the materials themselves. This phenomenon is
equally applicable among the discrete diamond crystals within the
matrix of polycrystalline diamond.
[0017] Since polycrystalline diamond is produced from discrete
natural or synthetic diamond crystals, each crystal boundary is
potentially a thermal barrier. However, when the diamond crystals
are intergrown at high pressure and high temperature their
boundaries are intimately merged creating an uninterrupted thermal
path through the material. Therefore, the more thorough the
diamond-to-diamond bonding, or intergrowth, the more efficiently
polycrystalline diamond is capable of conducting thermal
energy.
[0018] Another aspect of high thermal conductivity that the
applicants have discovered is that the lower the concentration of
nitrogen in the diamond and the larger the crystal size, the more
conductive the thoroughly intergrown polycrystalline diamond seems
to be. The applicants believe that this may be due to fewer defects
in the crystal lattice that affect phonon transport and to the
actual reduction of crystalline surface area, and thus the
boundaries, within the polycrystalline diamond matrix. Natural or
synthetic diamond is suitable for this invention, but synthetic
diamond having controlled growth characteristics and low nitrogen
content is preferred. The applicants have found that by mixing
large crystals, say up to 3000.mu., having low nitrogen content
into a matrix of smaller crystals, say greater than 10.mu., higher
thermal conductivity has been achieved. Another benefit from using
larger crystals is that the matrix may be made thicker, which also
contributes to the spreading capacity of the heat spreader. By
using large diamond crystals in the matrix, a heat spreader having
at least a cross section of 19 mm.times.at least 4 mm may be
achieved. This is especially important as heat densities continue
to increase.
[0019] Like other forms of diamond, high-pressure high-temperature
polycrystalline diamond is chemically inert and, therefore,
difficult to bond to other materials. This invention overcomes this
obstacle by providing an economical bondable surface integral to
the matrix that is formed in situ during the high-pressure
high-temperature sintering process.
[0020] The key ingredient is the formation of a layer of bondable
material on the surface of the heat spreader. The formation of that
surface is the distinguishing feature of the invention. Contrary to
prior art products where the aim is to consume all of the catalyst
in the process, the present invention seeks to pool the excess
catalyst, or other bondable material on the surface opposite the
substrate, and to use that layer as a bondable surface.
[0021] Since diamond is chemically inert, it resists chemical
adhesion to another material, making it difficult to obtain the
intimate contact required for efficient thermal transfer.
Polycrystalline diamond (PCD) is a product that exhibits diamond
crystals bonded together using a metal catalyst, and the finished
product contains some metal at interstitial sites on the surface,
but it is desirable that the presence of metal in the product be
kept to a minimum. Therefore, normally the interstitial material is
insufficient for a competent bond. The object of this invention,
then, is to produce a PCD product that has an intimate layer of
bondable material on at least one major surface. Such a bondable
surface may be composed of the catalyst material, itself, or the
material may be inert to the process.
[0022] A product according to the present invention may be produced
by obtaining a known quantity of a mixture of diamond fine powders,
on the order of from about 10.mu. to about 3000.mu. in size, a
known catalyst, such as cobalt, a cemented metal carbide substrate,
such as tungsten carbide, and a bondable material, such as copper.
Other materials that may be considered as the bondable are: Fe, Co,
Ni, Pd, Pt, Cr, Mo, W, Nb, Ta, Hf, Zr, Ti, V, Al, Si, Ga, In, Au,
Cd, Ag, An, Mg, Sn, Ge or compounds, carbides, or alloys thereof.
The cobalt catalyst may be supplied by infiltration from the
tungsten carbide substrate, although a slight amount of cobalt may
be mixed into the diamond power to complement the infiltrating
catalyst. If the catalyst is supplied by intermixing with the
diamond powder, it usually does not exceed 50 weight-percent of the
diamond powder and in most cases is less than 30 weight-percent,
The copper, or other bonding material, may be in foil or powder
form in an amount not to exceed the weight of the diamond matrix.
The bondable material may be less than 20 weight-percent of the
diamond mixture, as long as it is sufficient to produce the
bondable surface. The specific weight percent of bonding material
used in the process will depend upon the specific application: the
grain size of the diamond powders, the thickness of the diamond
matrix desired, the type of bonding material and type of surface to
which it will be bonded, and the processing conditions of time,
temperature, pressure are determinative of the amount material that
must be used. When a foil is used, it should be between about
0.002" to 0.050" inches thick. The foil is laid in the bottom of a
refractory metal container and the diamond mixture is then placed
on top of the foil. When a powder is used, it, too, is spread in
the bottom of the container. The powder may also be intermixed with
the diamond mixture. A tungsten carbide disk, or substrate, is then
placed on top of the diamond mixture. These components are then
compacted together inside the container and hermetically sealed
from contamination. The metal container is then placed inside a
reaction vessel known in the art, and the assembly is inserted into
an ultra high-pressure high-temperature press apparatus. The press
is activated and the diamond mixture, substrate, and bondable
material are taken to conditions where diamond is thermodynamically
stable. The specific parameters of the process are apparatus
dependent. That is to say, that there are several styles of HPHT
presses in use today and each style is distinguishable by its
design, method of operation, and its mechanical and hydraulic
systems. Generally, however, the process will take approximately 2
to 45 minutes. In every style of press, the reaction vessel
appropriate for the style of press being used, which includes a
resistance heating mechanism, is exposed to conditions of pressure
and temperature of approximately 50 to 70 Kilobars and between
about 1200.degree.-1500.degree. C., respectively. Under such
conditions, the catalyst in the substrate melts and a portion of it
infiltrates the diamond mixture causing the diamond crystals to
chemically bond to one another in a process known as intergrowth.
The copper foil also melts and back infiltrates a thin layer of the
mixture providing a intimate bond with the diamond matrix.
[0023] Although the copper is not a catalyst for diamond
intergrowth, it does not completely block the infiltrating cobalt
and does not completely prohibit intergrowth. A powder layer acts
much the same way as the copper foil. The advantage of the copper
foil is found in its ease of handling. When the copper powder is
intermixed with the diamond mixture, the melted copper is swept
from the diamond matrix by the infiltrating cobalt and allowed pool
on the opposing surface. Some of the copper is allowed to remain in
the matrix in order to strengthen the bond between the diamond and
the copper layer. In these three examples, the objective is to
provide a layer of bondable material on the major surface. The
reaction vessel is then cooled and returned to atmospheric
conditions and removed from the press.
[0024] Depending upon the type of bonding material used, the
diamonds at the interface between the bonding material are not
unaffected by the presence of the non-catalytic material may
exhibit a less competent pattern of intergrowth. Notwithstanding
the presence of the non-catalytic material, the copper layer is
intimately bonded to the diamond and creates a surface to which a
thermal source or heat sink may be attached.
[0025] An examination of the finished product reveals that the
diamond has intimately bonded to the substrate and the copper is
bonded to the diamond, forming a surface to which the thermal
components may be bonded. It is desirable that the layer of
bondable material be approximately between 0.002" to 0.020" thick.
The examination also reveals that the catalyst metal has collected
at interstitial sites throughout the matrix. These pools of
catalyst are less thermally conductive than the path formed by the
intergrown diamond, and, therefore, it is preferred to control the
amount of residual catalyst in the final product. Although, copper
is used in this example, other bondable materials as described
herein may be beneficially used.
[0026] The infiltrating cobalt may also form a bondable layer. This
may be achieved by altering the grain size of the diamond powder,
the amount of cobalt in the mixture, and the processing parameters.
Generally, the finer the diamond powder the more difficult it is to
infiltrate and the larger volume of catalyst required. However,
finer crystal sizes produce a more thermally conductive part since
the diamond matrix may be made denser. Therefore, a balance must be
achieved between particle size and volume of catalyst used in the
process. Also by accelerating the process so that the melting
temperature of the catalyst is reached in a shorter period of time
encourages swifter infiltration and promotes pooling of the
catalyst on the opposing surface.
[0027] The heat spreader of the present invention may be made to
have high electrical resistance. Diamond is not electrically
conductive; it is the residual metal catalyst in the PCD that
produces electrical conductivity. Therefore, in order to make the
heat spreader electrically resistive, it is necessary to remove the
residual metal from the diamond matrix. This is usually
accomplished by acid leaching in boiling aqua regia. The surface
not infiltrated with bondable material may be made more resistive
by masking off the bondable layer to protect it from acid, or by
partial immersion of the unbound layer into the acid. As the metal
is dissolved in the acid, the remaining diamond matrix becomes
electrically resistive. It is not necessary to remove all the metal
as long as there is a contiguous layer free of metal in the diamond
matrix. Even with the catalyst metal substantially removed, the
diamond matrix remains intact because the diamond crystals are
bonded, intergrown, together.
[0028] Another, method of producing a bondable layer on the heat
spreader matrix of the present invention is by intermixing with the
diamond crystals a bondable material selected from the periodic
table of elements consisting of Fe, Co, Ni, Pd, Pt, Cr, Mo, W, Nb,
Ta, Hf, Zr, Ti, V, Al, Si, Ga, In, Au, Cd, Ag, An, Mg, Sn, Ge or
compounds, carbides, or alloys thereof prior to high-pressure
high-temperature processing. The intermixing may be accomplished
either by mixing, milling, or coating the discrete crystals by such
standard art processes as sputter coating, ion implantation, or
physical or chemical vapor deposition. During the high-pressure
high-temperature sintering process, the bondable material is also
swept through the matrix by the infiltrating catalyst and
accumulates, or pools, in a layer near or on the opposite surface
of the heat spreader. The thickness of this bondable layer need
only be sufficient to form an acceptable bond and may vary
depending on the volume of bondable material in the matrix and is
desirably about 1 mm, preferably 0.25 mm, or more preferably 0.025
mm. After processing, the bondable material may be machined to a
desired configuration and thickness, and to remove surface
asperities. A thin bondable layer is preferred over a thick
bondable layer because thermal impedance is proportional to the
width of the junction between the heat spreader and the IC die or
the heat sink.
[0029] Another method of producing the bondable layer on the heat
spreader of this invention is by positioning the bondable material,
either as a powder or as a solid foil or sheet, adjacent to the
diamond crystals prior to high-pressure high-temperature
processing. Then as the temperature increases, the bondable
material melts and infiltrates the diamond mass, combining with the
sweeping catalyst material and becoming integral to the heat
spreader.
[0030] Additionally, even the metal carbide substrate on which the
diamond crystals are mounted may be used in some cases as the
bondable layer. In the high-pressure high-temperature sintering
process the metal carbide substrate also becomes intimately bonded
to the diamond mass. After processing the metal carbide substrate
may be ground or lapped thin--also producing a bondable surface on
one side of the heat spreader.
[0031] In applications where the heat spreader is mounted adjacent
the IC die, it may be desirable to make the heat spreader
electrically resistive. Diamond is electrically non-conductive.
But, the residual catalyst material in the matrix of
polycrystalline diamond causes it to be electrically conductive.
Electrical resistance may be achieved, however, by either
chemically or mechanically removing at least a portion of the
residual catalyst material from at least a portion of the surface
adjacent the heat source, creating an insulating layer of diamond.
Using this method a substantial portion of the catalyst also may be
removed from the entire matrix if so required. The applicants have
found that where there is thorough diamond intergrowth the removal
of the residual catalyst material from the matrix does not
significantly compromise the structural integrity of the heat
spreader.
[0032] FIG. 1 is a perspective illustration of a heat spreader of
the present invention. It depicts a matrix (15) of high-pressure
high-temperature polycrystalline diamond that is naturally black in
color. The black color is an added intrinsic benefit for the heat
spreader application; the back color enhances the thermal
absorption. The heat spreader has bondable material layers (11 and
1) on each of its major contact surfaces that permit intimate
attachment of the heat spreader to a thermal source as well as to a
heat sink. According to the present invention, these bondable
contact surfaces are economically produced in situ during the
high-pressure high-temperature process and are integral to the
polycrystalline diamond matrix. Each contact surface may be
composed of different materials. For example, the junction layer
(11) may be produced by lapping or grinding the metal carbide
substrate to a thin layer, while the junction layer (1) may be
produced by providing the bondable material in the form of a powder
of sheet positioned adjacent the diamond matrix. Another method may
be to sandwich the diamond crystals between two metal carbide
substrates, and after processing, by then lapping or grinding both
sides of the heat spreader to produce the bondable surface. These
contact surfaces may be further modified by standard machine shop
art of lapping and grinding in order to remove asperities and
achieve low surface roughness and high flatness. Although the
configuration shown is a right cylinder, different prismatic
configurations not shown are also producible and intended within
the scope of the present invention.
[0033] FIG. 2 is a section illustration of a heat spreader of the
present invention depicting the metal carbide substrate (17)
intimately bonded to the polycrystalline diamond matrix (23).
Bondable material is shown in high concentration layers along the
major contact surfaces (21 and 25) of the heat spreader. The metal
carbide substrate (17) may be reduced to a thin contact surface
(19) by employing standard machine shop art, or it may be removed
altogether exposing the junction surface (21) for bonding either to
the thermal source or the heat sink. As described earlier in this
application, the bondable material depicted in high concentrations
along the junction surfaces (21 and 25) may be provided by
intermixing, by coating the diamond crystals, or by positioning a
powder or sheet of the bondable material adjacent the diamond prior
to high-pressure high-temperature processing. The major contact
surfaces at 19 and 21 are depicted having a non-planar, convex
profile. Though not shown, other non-planar profiles are intended
within the scope of this invention, such as concave, conical,
truncated conical, and hemispherical.
[0034] FIG. 3 is another section illustration of the present
invention depicting an application where the metal carbide
substrate (27) is attached to the diamond matrix (31) at an
interrupted interfacial surface (29) having a layer of bondable
material (30) along the interface. This sample is also provided
with a bondable layer (33) on the opposite major contact surface of
the heat spreader. The interrupted profile of the interface (29)
may be the result of grooves in either a parallel or radial pattern
along the surface of the diamond matrix. Or it may be the result of
discrete protrusions, such as nodes, projecting above the
spreader's major contact surface. Additionally, these
configurations may be combined with the non-planar profile of FIG.
2 to present a complex non-planar profile that greatly increases
the versatility of this invention. The added surface created by the
non-planar configurations enhances the means of attachment and the
spreading capacity of the polycrystalline diamond.
[0035] FIG. 4 is a magnified section illustration of the
high-pressure high-temperature polycrystalline diamond matrix of
the present invention. High-pressure high-temperature
polycrystalline diamond is composed of discrete diamond crystals
that are chemically intergrown at high pressure and high
temperature in the presence of a metal catalyst. In this
illustration, intergrown diamond crystals (37) are vaguely bordered
at interstitial sites (41) by residual metal catalyst. The
intergrown structure of the matrix creates an uninterrupted thermal
path (39) for transmitting heat from the thermal source to the heat
sink. Since diamond has low thermal capacity, heat accumulation in
the matrix is slight and quickly transmitted into the heat sink.
The accumulation of bondable material (35) along the major contact
surfaces of the heat spreader, according to the present invention,
provides a low-impedance junction to both the heat source and the
heat sink. The residual metal catalyst, which is electrically
conductive, may be removed from the contact surfaces of the matrix,
or from the matrix itself, by chemical or mechanical means in order
to produce an insulating medium. The removal of the residual metal
catalyst does not significantly affect the structural integrity of
the matrix. The matrix of the present invention is strong, rigid,
and thermally compatible with both the silicon of the IC die and
the ceramic or metal composition of the heat sink, reducing the
likelihood that the thermal cycling of the IC will result failure
of the heat spreader or the die.
[0036] FIG. 5 is a section illustration of a heat spreader of the
present invention incorporated into a thermal management system.
The heat spreader (53) is mounted onto a circuit board (49)
positioned between the IC die (47) and the heat sink (43). The heat
spreader of the present invention is intimately bonded to the heat
sink and the IC die along its major contact surfaces using the
bondable material layers (45) of the present invention. Low thermal
impedance is achieved at the junctions by maintaining the bonding
layer sufficiently thin so as to reduce contact surface thermal
resistance. As the computer IC is cycled hot spots (51) develop on
the surface of the die. The thermal energy radiating from the die
is quickly spread across the surface of the die and transmitted
into the heat spreader. Since the heat spreader has low thermal
capacity, heat does not accumulate in the spreader itself, but is
quickly conducted into the high thermal capacity heat sink and
convectively transmitted into the atmosphere or other high capacity
medium. Adaptations of this design are obvious to those skilled in
the art and are included within the scope of this invention
although not depicted. For example, a free flowing or jetted fluid
or cryogenic thermal management system may be substituted for the
heat sink shown. Also, heat pipes or cooling fluid conduits (55 and
57) may be added to the heat spreader or the heat sink in order to
increase the thermal differential, adding to the efficiency of the
system. Or, the heat spreader may be positioned on both sides of
the IC die to further enhance dissipation of thermal energy.
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