U.S. patent application number 10/554496 was filed with the patent office on 2006-11-23 for composite material, electrical circuit or electric module.
Invention is credited to Ernst Hammel, Jurgen Schulz-Harder.
Application Number | 20060263584 10/554496 |
Document ID | / |
Family ID | 33394381 |
Filed Date | 2006-11-23 |
United States Patent
Application |
20060263584 |
Kind Code |
A1 |
Schulz-Harder; Jurgen ; et
al. |
November 23, 2006 |
Composite material, electrical circuit or electric module
Abstract
The invention relates to a novel composite material, especially
for applications in the field of electrical engineering. Said novel
material has a thermal coefficient of expansion that is smaller
than 12.times.10.sup.-6 K.sup.-1 in at least two axes of a
three-dimensional system that are perpendicular in relation to each
other.
Inventors: |
Schulz-Harder; Jurgen;
(Lauf, DE) ; Hammel; Ernst; (Wien, AT) |
Correspondence
Address: |
Hoffman, Wasson & Gitler
2461 South Clark Street
Suite 522 Crystal Center 2
Arlington
VA
22202
US
|
Family ID: |
33394381 |
Appl. No.: |
10/554496 |
Filed: |
April 20, 2004 |
PCT Filed: |
April 20, 2004 |
PCT NO: |
PCT/DE04/00824 |
371 Date: |
October 26, 2005 |
Current U.S.
Class: |
428/292.1 ;
148/432; 148/437; 257/E23.074; 257/E23.106; 257/E23.11 |
Current CPC
Class: |
H01L 2924/3025 20130101;
H01L 2224/85399 20130101; H01L 2224/45099 20130101; H05K 3/0058
20130101; H01L 23/373 20130101; H01L 2224/05599 20130101; H01L
2224/85399 20130101; H01L 2924/01019 20130101; H01L 2924/09701
20130101; H01L 2924/00014 20130101; H01L 2224/05599 20130101; B22F
2999/00 20130101; B22F 2998/10 20130101; H01L 2924/00014 20130101;
Y10T 428/249924 20150401; H01L 2924/0102 20130101; H01L 2224/48227
20130101; H05K 3/38 20130101; H01L 2924/01322 20130101; H01L
2924/01079 20130101; H01L 23/3735 20130101; B22F 2998/10 20130101;
H01L 23/49877 20130101; H05K 1/0306 20130101; H01L 24/48 20130101;
H01L 2924/00014 20130101; H01L 2224/48091 20130101; C22C 49/14
20130101; C22C 2204/00 20130101; H01L 2224/45099 20130101; B22F
1/004 20130101; B22F 2999/00 20130101; H01L 2224/45015 20130101;
C22C 47/08 20130101; B22F 1/0018 20130101; H01L 2924/00014
20130101; H01L 2924/00014 20130101; C22C 49/14 20130101; C22C 47/06
20130101; H01L 2924/207 20130101; H01L 2924/00014 20130101; H01L
2224/48091 20130101 |
Class at
Publication: |
428/292.1 ;
148/432; 148/437 |
International
Class: |
C22C 49/14 20060101
C22C049/14; C22C 9/00 20060101 C22C009/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 8, 2003 |
DE |
103208380 |
Claims
1. A composite material or composite raw material, comprising: a
matrix of at least one metal or metal alloy, at least one ceramic
and/or one glass and nanofibers with a thickness between
approximately 1.3 nm to 300 nm, with a length/thickness ratio for
the most part greater 10.
2. A composite material according to claim 1, wherein if the
composite material is embodied as a fiber-reinforced
metal-ceramic-glass composite material as a substrate for electric
applications for thermal management, consisting of a carrier
substrate based on ceramic or glass materials and of at least one
fiber-reinforced metal layer applied on one side, the fibers in the
metal layer consist of carbon nanotubes, which have a thickness of
1.3 to 300 nm and a length/thickness ratio of greater 10, and the
content of nanofibers in the metal matrix is between 10 and 70
percent by volume.
3. A composite material according to claim 2, wherein the carrier
substrate contains nanofibers made of boron nitride and/or tungsten
carbide.
4. A composite material according to claim 1 wherein the thermal
expansion coefficient of the material in at least two perpendicular
spatial axes is less than 12.times.10.sup.-6K.sup.-1, and/or the
thermal conductivity of the composite material at least in a
partial area is greater than that of the metal or metal alloy.
5. A composite material according to claim 1, wherein the thermal
conductivity of the composite material at least in a partial area
is greater than that of copper.
6. A composite material according to claim 1, wherein the
nanofibers are distributed isotropically or nearly isotropically in
their orientation at least in the at least two spatial axes.
7. A composite material according to claim 1, wherein at least part
of the nanofibers are nanotubes.
8. A composite material according to claim 1, wherein the
nanofibers are made of an electrically conductive material.
9. A composite material according to claim 1, wherein the
nanofibers are made of carbon and/or boron nitride and/or tungsten
carbide.
10. A composite material according to claim 1, wherein the ceramic
is made of aluminum nitride and/or aluminum oxide and/or silicon
nitride.
11. A composite material according to claim 1, wherein the metal is
copper or a copper alloy.
12. A composite material according to claim 1, wherein the metal is
aluminum or an aluminum alloy.
13. A composite material according to claim 1, wherein the
nanofibers are provided in a matrix formed by the at least one
metal or the at least one metal alloy.
14. A composite material according to claim 1, wherein the
nanofibers are provided in the ceramic and/or in the glass.
15. A composite material according to claim 1, wherein ceramic
particles and nanofibers are provided in the matrix formed by the
at least one metal or the at least one metal alloy.
16. A composite material according to claim 1, wherein the content
of the nanofibers in the matrix of the at least one metal or metal
alloy is approximately 10-70 percent by volume.
17. A composite material according to claim 1, further comprising a
perform made of the nanofibers, into which the at least one metal
or metal alloy is applied through melt infiltration.
18. A composite material according to claim 1, wherein the matrix
of the at least one metal or the at least one metal alloy with the
nanofibers is produced using an HIP process.
19. A composite material according to claim 1, wherein the matrix
of the at least one metal or the at least one metal alloy and the
nanofibers is produced through electrolytic and/or chemical
precipitation of the metal or of the metal alloy on the nanofibers
or a perform made of the nanofibers.
20. A composite material according to claim 1, wherein the matrix
of the at least one metal or the at least one metal alloy and the
nanofibers is produced through electrolytic and/or chemical
precipitation of the metal or of the metal alloy and the nanofibers
on a perform made of metal or a metal alloy or of ceramic.
21. A composite material according to claim 1, further comprising
its embodiment as a laminate with at least two interconnected
material sections or layers forming said laminate.
22. A composite material according to claim 21, wherein at least
one material section is made of ceramic, and at least one
additional material section is made of the at least one metal or
the at least one metal alloy.
23. A composite material according to claim 22, wherein the at
least one material section made of ceramic contains the
nanofibers.
24. A composite material according to claim 22, wherein the at
least one material section made of the at least one metal or of the
at least one metal alloy contains the nanofibers.
25. A composite material according to claim 22, wherein the
material sections are bonded together by soldering, for example by
the active soldering process.
26. A composite material according to claim 22, wherein the
material sections are bonded together by direct bonding, for
example by the DCB process.
27. A composite material according to claim 22, wherein the
material sections are bonded together by adhesive bonding.
28. A composite material according to claim 22, wherein the
material section made of the at least one metal or of the at least
one metal alloy comprises several elements or several layers.
29. A composite material according to claim 1, further comprising
its embodiment as a ceramic-metal substrate or as a printed circuit
board with at least one insulating layer formed by the ceramic and
with at least one metallization or metal layer formed by the metal
or metal alloy on at least one surface of the ceramic layer,
wherein the metal or the metal alloy and/or the ceramic contains
the nanofibers.
30. A composite material according to claim 29, wherein the
metallization forms strip conductors and/or contact surfaces and/or
fastening surfaces on at least one surface of the ceramic
layer.
31. A composite material according to claim 30, wherein the metal
layer is structured in order to form the strip conductors and/or
contact surfaces and/or fastening surfaces.
32. A composite material according to claim 29, wherein the at
least one metallization or metal layer is connected with an
additional element made of metal or of the metal alloy, and that
the additional element contains the nanofibers.
33. A composite material according to claim 1, further comprising
its embodiment as a component for thermal dissipation, as a heat
sink or as a housing or as part of a housing.
34. Electric circuit or electric module with at least one substrate
and with at least one electric component, wherein the substrate
consists at least partially of a composite material according to
one of the foregoing claims.
Description
[0001] The invention relates to a composite material or a composite
raw material according to the preamble of claim 1 and to an
electric circuit or an electric module according to the preamble of
claim 32.
[0002] A "composite material" or "composite raw material" according
to the present invention is generally a material comprising several
material components, for example in a common matrix or also at
least partially in at least two adjacent material sections that are
bonded together.
[0003] A "component for thermal dissipation" or a "heat sink"
according to the invention are generally components that are used
in electronics and particularly in power electronics and are used
to dissipate heat loss and to cool electric or electronic
components, for example base plates and/or thermal dissipation or
cooling plates in electric circuits or modules, substrates for
electric or electronic components, housings or housing elements of
electric components or modules, and also, for example, coolers,
heat pipes or elements of such active heat sinks through which a
coolant flows, such as water.
[0004] In many areas of technology, composite materials are used as
materials for constructions, components, etc., particularly when
material properties are required that cannot be achieved with a
single material component. The desired properties for the composite
material can be optimized by carefully selecting the individual
components and the physical and/or chemical properties of these
components, for example the thermal properties.
[0005] "Materials for Thermal Conduction", Chung et al., Appl.
Therm. Eng., 21, (2001) 1593-1605, gives a general overview of
materials for thermal conducting or thermal dissipating materials.
The article outlines the properties of possible individual
components and relevant examples for the composite materials.
[0006] Ting et al reports in J. Mater. Res., 10 (6), 1995,
1478-1484 on the manufacture of aluminum VGCF (Vapor Grown Carbon
Fiber) composites and their thermal conducting properties. U.S.
Pat. No. 5,814,408 Ting et al is the resulting patent specification
for the AL-VGCF MMC.
[0007] Composites with Carbon Fibrils.TM., a defined CVD carbon
fiber, in both a metal and polymer matrix are mentioned in U.S.
Pat. No. 5,578,543 Hoch et al.
[0008] Ushijima et al describes in U.S. Pat. No. 6,406,790 the
manufacture of a composite material with a special variant of CVD
grown carbon fibers as a filling material by means of pressure
infiltration of the matrix metal.
[0009] Houle et al report in U.S. Pat. No. 6,469,381 on a
semiconductor element that dissipates the heat produced during
operation by including carbon fibers in the substrate.
[0010] The use of coated carbon fibers in composite materials with
a metallic matrix is described by Bieler et al in U.S. Pat. No.
5,660,923.
[0011] Al.sub.2O.sub.3 fibers in an Al matrix and the manufacture
of corresponding fiber-reinforced composite materials is described
in U.S. Pat. No. 6,460,497, McCullough et al.
[0012] Due to the improved electrical properties, the use of
metal-ceramic substrates as printed circuit boards, for example
substrates made of aluminum oxide (Al.sub.2O.sub.3) or increasingly
also substrates made of aluminum nitride (AlN) is known in
particular for power modules, which are increasingly being used in
electric drive systems, for example in traffic and automation
technology. Layers or base plates made of copper, which have high
thermal conductivity and therefore are suitable for dissipating
power or heat loss and also for thermal spreading, have heretofore
been used for the substrate or transition layer to a heat sink,
which often has to dissipate a considerable power loss from such a
power module.
[0013] The disadvantage of this is the high fluctuation in the
thermal expansion coefficients of the materials used, namely of the
ceramic, the copper and also of the silicon of the active electric
or electronic components of such a module. Such power modules and
their components are subjected to a considerable change in
temperature not only during the manufacturing process, but also
during operation, for example during the transition from the
operating phase to the dwell or not operating phase and vice versa,
and also when the module is switched during operation. Due to the
differing expansion coefficients, these temperature changes cause
mechanical stress in the module, i.e. mechanical stress between the
ceramic and the adjacent metallizations or metal layers (such as
base plate on one side of the ceramic layer and strip conductors,
contact surfaces, etc. on the other side of the ceramic layer), and
also between metal surfaces and the electric or electronic
components located thereon, in particular semiconductor elements.
Frequent mechanical alternating stress causes material fatigue and
therefore failure of the module or its components.
[0014] This problem is compounded by the additional factor of
miniaturization and by the ensuing increase of the power density of
power modules. The thermal expansion coefficients of the material
components of a power module with a copper-ceramic substrate are
within the range of =16.8.times.10.sup.-6K.sup.-1 for copper and
=3.times.10.sup.-6K.sup.-1 for silicon.
[0015] Reference is also made to the following table, in which the
thermal conductivity and the thermal expansion coefficient are
specified for various materials. TABLE-US-00001 .quadrature..sub.th
in W/mK .quadrature. in 10.sup.-6/K Ag 428 19.7 Cu 395 16.8 CuCo0.2
385 17.7 CuSn0.12 364 17.7 Au 312 14.3 Al 239 23.8 BeO 218 8.5 AlN
140-170 2.6 Si 152 2.6 SiC 90 2.6 Ni 81 12.8 Sn 65 27 AuSn20 57
15.9 Fe 50 13.2 Si.sub.3N.sub.4 10-40 3.1 Al.sub.2O.sub.3 18.8 6.5
FeNi42 15.1 5.1 Silver epoxy cement 0.8-2 53 Epoxide molding
0.63-0.76 18-30 SiO.sub.2 0.1 0.5 W 130 4.5 Mo 140 5.1 Cu/Mo/Cu 194
6.0 AlSiC 160-220 7-10
[0016] Since the thermal conductivity for dissipation of the power
loss is necessary, the metals used especially in semiconductor
modules or their substrates for the metallizations, the base plate,
etc. must be able to conduct heat sufficiently. At present,
materials with a copper or aluminum base, such as Cu--W, Cu--Mo or
Al--SiC are preferred especially for heat sinks.
[0017] The method of manufacturing the metallization required for
strip conductors, connections, etc. on a ceramic, for example on an
aluminum-oxide ceramic, using the direct copper bonding technology
is known in the art, the metallization being formed by metal or
copper foils or metal or copper sheets, featuring on the top side a
layer or coating (melt layer) from a chemical bond with the metal
and a reactive gas, preferably oxygen. In the method described for
example in U.S. Pat. No. 3,744,120 or in DE-PS 23 19 854, this
layer or coating (melt layer) forms a eutectic with a melting
temperature below the melting temperature of the metal (e.g.
copper), so that when the foil is placed on the ceramic and all
layers are heated they are bonded together, namely through melting
the metal or copper essentially only in the area of the melt layer
or oxide layer.
[0018] This DCB method then comprises the following steps:
[0019] oxidation of a copper foil in a manner that results in an
even copper oxide layer;
[0020] placing of the copper foil on the ceramic layer;
[0021] heating of this composite or strukture to a process
temperature between approximately 1025 and 1083.degree. C., e.g. to
approximately 1071.degree. C.;
[0022] cooling to room temperature.
[0023] The object of the invention is to provide a composite
material, which while maintaining a high degree of thermal
conductivity, which is greater than or at least equal to that of
copper or copper alloys, has a thermal expansion coefficient
significantly lower than that of copper. This object is achieved by
a composite material according to claim 1. An electric circuit or
an electric module is exemplified according to claim 32.
[0024] The composite material according to the invention, which is
suitable for example for electrical engineering applications and
therefore for applications as a substrate or as a component for
dissipating heat in electric power modules, consists therefore
essentially of three main components, namely of at least one metal
or at least one metal alloy, of at least one ceramic and of
nanofibers, which have a thickness between 1.3 nm and 300 nm, and
the length/thickness ratio for a majority of the nanofibers
contained in the composite material being greater than 10. The
ceramic content can be replaced partially or entirely by glass, for
example by silicon oxide.
[0025] The nanofibers used effect the desired reduction of the
thermal expansion coefficient of the composite material in at least
two perpendicular spatial axes, preferably in all three
perpendicular spatial axes.
[0026] In the embodiment of the composite material according to the
invention, the following measures are possible in further
embodiments of the invention:
[0027] The nanofibers are distributed isotropically in their
orientation at least in the at least two spatial axes.
[0028] At least some of the nanofibers are for example nanotubes,
which are especially stable in axial direction, so that they
contribute very effectively to the desired reduction of the thermal
expansion coefficient.
[0029] The nanofibers preferably are made of an electrically
conductive material, so that the composite material comprising the
nanofibers or the part of the composite material comprising the
nanofibers can also be used for electric strip conductors or
contacts, etc., i.e. it possesses the necessary electric
conductivity for this application.
[0030] The nanofibers are preferably such made of carbon and/or of
boron nitride and/or of tungsten carbide. Other materials or
composites that are suitable for the manufacture of nanofibers are
also conceivable, in particular nanofibers made of carbon and
coated with boron nitride and/or tungsten carbide.
[0031] The ceramic used for the composite material according to the
invention is preferably an aluminum oxide ceramic or an aluminum
nitride ceramic, in which case the aluminum nitride ceramic is
characterized by especially high electric strength and by increased
thermal conductivity.
[0032] The metal component used for the invention is preferably
copper or a copper alloy. This applies in particular in the event
that the composite material is to be used for substrates or printed
circuit boards or as a component for thermal dissipation in
electric circuits or modules. Copper, and also copper alloys, are
relatively easy to process, in particular when this material
component of the composite material contains the nanofibers.
[0033] It is also possible to provide the nanofibers in the at
least one metal or the at least one metal alloy and/or in the
ceramic and/or in the glass, for example in a matrix formed by the
metal or metal alloy.
[0034] The nanofiber content of the composite material is for
example between 10 and 70 percent by volume, preferably between 40
and 70 percent by volume, in relation to the total volume of the
material component of the composite material containing said
fibers.
[0035] If the nanofibers are contained in the metal or in the metal
alloy of the composite material, then a wide range of methods are
available for implementing this special design. It is possible, for
example, to first form a perform from the nanofibers, for example
in the form of a three-dimensional latticework, a fleece-like
structure, a hollow body or tubular structure, etc., wherein the at
least one metal or the at least one metal alloy is incorporated
into this perform. A wide variety of methods is conceivable for
this design in particular, for example through chemical and/or
electrolytic precipitation, through melt infiltration, etc.
[0036] According to one embodiment of the invention, the composite
material is a fiber-reinforced ceramic-glass composite as a
substrate for electric or electronic applications and consists of a
carrier substrate based on ceramic and/or glass materials and at
least one fiber-reinforced metal layer applied to one side. The
fibers in the metal layer are then for example carbon nanotubes,
with a thickness of 1.3 to 300 nm and a length/thickness ratio
>10, and the nanofibers are present in the metal matrix of the
metal layer with a content of 10 to 70 percent by volume. If the
carrier substrate also contains nanofibers, then they have a high
nitride and/or tungsten carbide content.
[0037] Furthermore, it is possible to apply the metal and the
nanofibers to a perform or a substrate made of metal and/or
ceramic, for example through chemical and/or electrolytic
precipitation.
[0038] Other methods for manufacturing the matrix of the at least
one metal or the at least one metal alloy with the nanofibers are
conceivable, for example, the so-called HIP technology, in which
the at least one metal or the at least one metal alloy is inserted
into a capsule in powder form mixed with the nanofibers, after
which the capsule is tightly sealed with a cap. Afterwards, the
interior of the capsule is evacuated and sealed so that it is
gastight. Then pressure is applied to the entire capsule (e.g. gas
pressure using inert gas, for example argon, or hydrostatic
pressure) and therefore also to the material contained in the
capsule, while simultaneously heating it to a process temperature
between 500 and 1000.degree. C.
[0039] In a further process step, after cooling, the capsule and
the metal blank containing the nanofibers are separated, so that
the blank can be further processed, for example through machining
or cutting, sawing and/or rolling to manufacture boards or foils,
which then are bonded with a ceramic layer to manufacture a
metal-ceramic substrate or a printed circuit board.
[0040] Especially for use in electric or electronic components the
composite material according to the invention is designed as a
laminate, namely with at least two bonded material sections or
layers, where one material section or one layer is made of the at
least one metal or the at least one metal alloy and the other
material section or the other layer is made of ceramic. The
nanofibers are then contained for example in the at least one
material section made of the metal or the metal alloy. Generally it
is also possible that the nanofibers are likewise contained in the
ceramic, for example to reinforce the mechanical stability of the
ceramic and/or to improve the thermal conductivity of the
ceramic.
[0041] If the composite material consists of at least one material
section made of the at least one metal or the at least one metal
alloy and of the material section made of ceramic, then both
material sections or layers are bonded together, for example by
soldering, preferably by the active soldering process, or using the
known direct bonding technology.
[0042] Especially in the possible embodiment of the composite
material as a metal-ceramic substrate or printed circuit board, a
metallization is provided on at least one surface of one ceramic
layer, which (metallization) is formed by the at least one metal or
the at least one metal alloy and contains the nanofibers. This
metal layer is then for example the base plate of such a substrate
or is bonded with such a base plate, with which the substrate is
bonded with a passive heat sink, for example in the form of a
cooler body or with an active heat sink, for example in the form of
a cooler through which a coolant flows, also a micro cooler.
[0043] Strip conductors and/or contact surfaces and/or fixing or
fastening surfaces for components of an electric circuit or module,
for example, are then provided on the other surface of the ceramic
layer. The metal or metal alloy forming these strip conductors,
contact surfaces, etc. can also contain the nanofibers, in which
case the structured metallization of the strip conductors etc. is
effected in the normal manner, namely in that after application of
a metal layer, this layer is formed into the structured
metallization, for example through an etch-masking process.
[0044] Therefore, the invention is used to create a composite
material in which considerably higher conductivity (e.g. >380
W(mK).sup.-1) is achieved through the dispersion of the nanofibers
into the metal matrix, for example copper matrix, combined with
reduced thermal expansion. Furthermore, especially the use of
copper for the metal matrix ensures easy processing of the metal
containing the nanofibers, so that all standard processing methods,
such as drilling, milling, punching, and also chemical processing,
are possible.
[0045] The composite material according to the invention can be
used for solutions in the field of thermal management that
previously presented major difficulties, e.g. also in laser
technology, where in particular the differing thermal expansion
coefficients between the semiconductor material of a laser bar and
the metal of a heat sink considerably decreases the service life of
laser diodes or laser diode arrays. The improved thermal
conductivity can be used to achieve higher power densities than
previously possible in electric and electronic power modules,
namely with the possibility of miniaturization of electrical and
electronic modules and assemblies and with the possibility of
additional applications especially also in such technical fields in
which the miniaturization and ensuing reduction of mass and weight
is of consequence, as in air and space technology.
[0046] The composite material according to the invention makes it
possible to combine in one material properties that were previously
less than optimally compatible. If the nanofibers are provided in
the metal matrix, then they serve as reinforcing components, which,
with their high thermal conductivity (greater than 1000
W(mK).sup.-1) and their negligible thermal expansion coefficient,
significantly reduce the expansion coefficient of the overall
composite material and Improve its thermal conductivity.
[0047] The invention is described below in detail based on
exemplary embodiments with reference to the drawings, wherein:
[0048] FIG. 1 shows a simplified representation of an electric
power module with a composite material according to the
invention;
[0049] FIG. 2 shows a simplified schematic representation of the
various process steps (positions a-d) of the HIP process for
manufacturing a metal-nanofiber composite;
[0050] FIG. 3 shows a schematic representation of a process for
further processing of starting material containing the at least one
metal or the at least one metal alloy and the nanofibers;
[0051] FIGS. 4 and 5 show a schematic representation in side view
and in top view of a bath for electrolytic and/or chemical
co-precipitation of metal and nanofibers on a metal foil or
perform;
[0052] FIGS. 6 and 7 show a schematic representation in top view of
a bath for electrolytic and/or chemical co-precipitation of metal
on a perform formed by nanofibers.
[0053] FIG. 1 shows a simplified representation in side view of an
electric power module 1, which consists, inter alia, of a
ceramic-copper substrate 2 with various electronic semiconductor
components 3, of which only one power component is depicted for the
sake of clarity, and of a base plate 4. The copper-ceramic
substrate 2 comprises a ceramic layer 5, for example of aluminum
oxide or aluminum nitride ceramic, wherein different ceramics can
be used if the layer 5 is formed from multiple parts, and one upper
metallization 6 and one lower metallization 7. The metallizations 6
and 7 in the depicted embodiment are each formed by a foil, which
contains nanofibers in a matrix of copper or a copper alloy, for
example with a content of 10-70 percent by volume, in relation to
the total volume of the respective foil or metallization,
preferably with a content of 40-70 percent by volume.
[0054] The component 3 is a power semiconductor component, e.g. a
transistor for switching high currents, e.g. for controlling an
electric motor or a drive. Other power semiconductor components are
also conceivable, for example laser diodes. The thickness of the
base plate 4 in the axis direction perpendicular to the planes of
the metallizations 6 and 7 is a multiple of the thickness of the
foils used for these metallizations 6 and 7.
[0055] The two metallizations 6 and 7 are bonded two-dimensionally
with one surface of the ceramic layer 5 using a suitable method,
for example by means of DCB technology or the active soldering
process. Furthermore, the metallization 6 is structured in the
required manner, preferably using the etch-masking method known to
persons skilled in the art, in order to form strip conductors,
contact surfaces, fasting surfaces for fastening or soldering of
components 3, of shielding surfaces or strips functioning as
inductors, etc. Other methods are also conceivable, for example in
the manner that the structuring is produced by mechanical
processing of the foil forming the metallization 6, for example
following or preceding the application of the metallization 6 to
the ceramic layer 5. The foil forming the metallization 7 is not
structured in the depicted embodiment. In the depicted embodiment,
this foil covers a large part of the bottom of the ceramic layer 5,
wherein the edge area of the ceramic layer 5 is kept free from the
metallization 7 in order to increase the electric strength, i.e.
the edge of the metallization 7 ends at a distance from the edge of
the ceramic layer 5. Furthermore, the base plate 4 in the depicted
embodiment is designed so that its perimeter clearly protrudes
beyond the perimeter of the copper-ceramic substrate 2. The base
plate 4 is for example the base plate of a housing of the power
module not further depicted.
[0056] The metallization 7 is connected two-dimensionally on its
surface facing away from the ceramic layer 5 with the base plate 4,
using a suitable method, such as soldering, brazing or active
soldering, or likewise using DCB technology. The base plate 4 in
the depicted embodiment is likewise made of a metal or a metal
alloy, for example of copper or a copper alloy, wherein the metal
or the metal alloy of the base plate 4 likewise contains the
nanofibers with a content of 10-70 percent by volume relative to
the total volume of the base plate 4, preferably with a content of
40-70 percent by volume. The nanofibers in the metallizations 6 and
7 and in the base plate 4 are distributed isotropically or nearly
isotropically with respect to their orientation at least in the two
perpendicular spatial axes that define the planes of the
metallizations 6 and 7 and the plane of the top of the base plate 4
connected with the metallization 7.
[0057] The nanofibers have a thickness between 1.3 nm and 300 nm,
wherein the greater part of the nanofibers contained in the metal
matrix has a length/thickness ratio>10. The nanofibers in this
embodiment have a carbon base or are made of carbon, for example in
the form of nanotubes. Generally it is also possible, however, to
replace these nanofibers made of carbon in whole or in part with
nanofibers made of another suitable material, for example of boron
nitride and/or tungsten carbide. Generally the nanofibers can be
distributed Isotropically with respect to their orientation in all
three perpendicular spatial axes, i.e. in the two axes defining the
planes of the metallizations 6 and 7 and the top of the base plate
4 and in the axis extending perpendicular to the other two
axes.
[0058] The use of nanofibers in the matrix of the metal or metal
alloy significantly reduces the thermal expansion coefficients of
the metallizations 6 and 7 and in particular also of the base plate
4, especially in the axes of the preferred orientation of the
nanofibers, namely in the axes defining the planes of the
metallizations and the planes of the top of the base plate, to a
value of <5.times.10.sup.-6K.sup.-1, especially also in the
relevant temperature range for substrates of semiconductor modules,
i.e. between room temperature (approximately 20.degree. C.) and
250.degree. C. The electric conductivity especially of the strip
conductors formed by the metallization 6 corresponds to the
electric conductivity of copper or of a copper alloy without the
nanofibers.
[0059] The thermal conductivity of the metallizations 6 and 7 and
of the base plate 4 is greater than that of copper and is for
example on the order of =600 W(mK).sup.-1 or greater. Due to the
extremely reduced thermal expansion coefficient as compared with
pure copper or a copper alloy, it is clearly adapted to the thermal
expansion coefficient of the silicon of the semiconductor component
3, and also clearly adapted to the thermal expansion coefficient of
the ceramic of the ceramic layer 5. This significantly reduces
thermal stress, as a result of temperature changes in the power
module 1, between the metallization 6 and the silicon body of the
components 3 and the ceramic of the ceramic layer 5, and in
particular also thermal stress between the metallization 7
reinforced by the base plate 4 and the ceramic layer 5. Such
temperature changes are caused by the switching states of the power
module 1, and also by changes in power during operation of the
power module, for example by corresponding control of this
module.
[0060] The improved thermal conductivity as compared with copper
significantly improves the thermal dissipation of the heat loss
produced by the semiconductor component 3 and also significantly
improves the thermal spreading through the metallization 7 and
improves the transfer of the power loss to the base plate 4. The
latter is then connected with a passive heat sink, for example with
a cooler or radiator, which is located in a current of a medium
dissipating the heat loss, in the simplest case an air current, or
the base plate 4 is connected to an active heat sink, for example
with a micro cooler, through which a coolant flows, for example a
gaseous and/or vaporous and/or liquid coolant, for example water.
Furthermore it is possible to provide the base plate 4 on a
so-called heat pipe for the especially effective dissipation of the
heat loss from this base plate 4 to a passive or active cooler.
[0061] As an alternative to the embodiments described above, it is
also possible to design the base plate 4 as a cooler, in particular
as an active cooler, e.g. micro cooler, through which the coolant
flows, or also as a heat pipe. In these cases it is also
advantageous to manufacture a part of the cooler or heat pipe,
which (part) is connected to the metallization 7, from the metal
containing the nanofibers or from the corresponding metal
alloy.
[0062] FIG. 2 shows, in various process steps (positions a-d), a
possibility for manufacturing a starter material consisting of the
metal matrix and the nanofibers contained in this matrix. In this
method, which is also referred to as the HIP method, a powdered
mixture 8 of particles from the metal or metal alloy, for example
of copper or copper alloy, and of the nanofibers are inserted into
a capsule 9, so that this capsule 8 is filled to approximately 60%
of its volume with the mixture 8.
[0063] Mixing additives can also be added to the mixture 8,
especially in order to maximize the portion of nanofibers and to
achieve an even distribution of these fibers, inter alia, to reduce
the adhesion between the nanofibers. Furthermore, to improve the
bond between the metal, for example copper, and the carbon of the
nanofibers, it can be advantageous to use nanofibers with a
fishbone surface structure, which improves the mechanical bond. It
can also be advantageous to coat the nanofibers with reactive
elements, which cause a chemical bond, and/or to charge the
nanofibers with the metal and/or with ceramic and/or boron nitride
and/or tungsten carbide, for example by means of vapor deposition,
for example.
[0064] In a further process step (position b) a cap 10 is placed on
the upper opening of the capsule 9 and is bonded tightly with the
capsule, for example by welding.
[0065] In a further process step, the interior of the capsule 9 is
evacuated by means of a connection 11 provided on the cap 10 and
the interior of the capsule 8 is then sealed so that it is
gastight.
[0066] In a further process step (position d) the ductile, sealed
capsule 9 is subjected to high pressure on all sides at a process
temperature between 500 and 1,000.degree. C., for example. This
pressurization on all sides of the capsule 9 takes place in a
closed chamber 12 by means of hydrostatic pressure acting on the
capsule 9, as indicated in position d by the arrows there. This
actual HIP process causes a reduction in volume, resulting in
deformation of the capsule 9. As a rule, the loss of volume
occurring during this deformation is approximately 5-10%, but can
also be greater, for example as high as 20%. The capsule 9 and the
corresponding cap 10 and the connection between these two elements
is such that the capsule is not damaged. In order to calculate the
reduction behavior, the capsule 9 has a simple geometric shape and
thin walls.
[0067] After the HIP process, the capsule 9 and the starter
material manufactured for example as a block in the HIP process are
separated, so that the starter material can be further processed in
a suitable manner.
[0068] The capsule 9 and its cap 10 serve several functions in the
HIP process, namely as an enclosed space during the evacuation for
reduction of the open porosity in the powdered starter material,
for transfer of the hydrostatic pressure during the actual HIP
process and also for shaping the end product produced by the
method.
[0069] FIG. 3 shows, in various positions a-d, a possibility for
further processing of the end product 13 produced by the HIP
process. This is depicted as a block in FIG. 3 (position a). Using
a suitable roller mechanism 14 the product 13 is then formed into a
foil 15 (position b), which is then rolled for further use
(position c). Position d again shows that the foil 15 or
corresponding blanks from this foil can be applied, for example
using the DCB method or another suitable process, to the ceramic
layer 5 in order to form the metallizations 6 and 7, in which case
the metallization 6 is structured in further process steps not
depicted in FIG. 3.
[0070] FIGS. 4 and 5 show a further possibility for manufacturing
the starter or row material, which contains the nanofibers in the
metal matrix. In this process, metal or copper foils are arranged
in a suitable bath containing the nanofibers and the metal, for
example copper, from which (bath) copper and nanofibers are then
precipitated electrolytically and/or chemically onto the foil
blanks 16.
[0071] The starter material obtained from this process is then used
directly as a layer containing the metal or metal alloy together
with the nanofibers in a laminated embodiment of the composite
material according to the invention, for example for the
metallizations 6 and 7 or the base plate 4 of the power module 1 of
FIG. 1, or the (plate-shaped) starter material produced with this
process is subjected to further processing, for example rolling,
before it is used as a material component in the composite
material.
[0072] In deviation from the above description, it is possible in
the process of FIGS. 4 and 5 to provide one or more performs in the
bath 17, the perform being formed by a three-dimensional structure,
for example a network or a fleece-like structure made of
nanofibers, so that the precipitation of copper and additional
nanofibers from the bath 17 takes place on the respective perform
to form a material containing the nanofibers and the metal or
copper. For better bonding with the metal, the nanofibers of the
perform in this embodiment are also chemically pretreated with
reactive elements, which improve the mechanical bond between the
nanofibers and the metal, for example copper. The charging of the
nanofibers with the metal, for example by means of vapor
deposition, Is likewise conceivable in this process.
[0073] For the perform in the process in FIGS. 4 and 5, the ceramic
layer 5 itself can also be used, on which the metal (copper) and
the nanofibers are then precipitated electrolytically and/or
chemically from the bath 17. For this purpose, the ceramic layer 5
is first pretreated on its surfaces, on which the co-precipitation
of nanofibers and metal is to take place, for example electrically
conductive, e.g. by applying a thin metal or copper layer.
[0074] FIGS. 6 and 7 show as a further possible embodiment a
process in which copper is electrolytically and/or chemically
precipitated on performs 18, which are formed by interlocked
fibers, from a bath 19, which contains coppers or copper salts. The
product thus obtained can then be used as the starter material for
further processing. Furthermore, in particular with this embodiment
it is also possible to allow nanofibers or copper-coated nanofibers
to protrude from the material containing them, resulting in an
impurity resistant lotus effect and/or enabling control of wetting
effects of the material.
[0075] The invention was described above based on exemplary
embodiments. It goes without saying that numerous modifications and
variations are possible without abandoning the underlying inventive
idea on which the invention is based.
[0076] For example, it is possible with the power module 1 of FIG.
1 to manufacture only the base plate 4 and/or only one of the
metallizations 6 or 7 from the material containing the nanofibers.
Furthermore, it is also possible to provide nanofibers in the
ceramic layer 5, in order to increase the thermal conductivity of
the ceramic layer, for example.
REFERENCE NUMBERS
[0077] 1 power module [0078] 2 copper-ceramic substrate [0079] 3
power component [0080] 4 base plate [0081] 5 ceramic layer [0082]
6, 7 metallization [0083] 8 mixture [0084] 9 capsule [0085] 10 cap
[0086] 11 cap connection [0087] 12 chamber [0088] 13 starting
product of metal matrix with nanofibers [0089] 14 rolling mechanism
[0090] 15 foil [0091] 16 starter foil [0092] 17 bath for
co-precipitation of nanofibers and copper [0093] 18 perform [0094]
19 bath for precipitation of copper
* * * * *