US20060263584A1 - Composite material, electrical circuit or electric module - Google Patents

Composite material, electrical circuit or electric module Download PDF

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Publication number
US20060263584A1
US20060263584A1 US10/554,496 US55449605A US2006263584A1 US 20060263584 A1 US20060263584 A1 US 20060263584A1 US 55449605 A US55449605 A US 55449605A US 2006263584 A1 US2006263584 A1 US 2006263584A1
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Prior art keywords
metal
composite material
nanofibers
material according
ceramic
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US10/554,496
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Jurgen Schulz-Harder
Ernst Hammel
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Electrovac AG
Curamik Electronics GmbH
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Electrovac AG
Curamik Electronics GmbH
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Assigned to CURAMIK ELECTRONICS GMBH, ELECTROVAC AG reassignment CURAMIK ELECTRONICS GMBH ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: HAMMEL, ERNST, SCHULZ-HARDER, JURGEN
Publication of US20060263584A1 publication Critical patent/US20060263584A1/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L23/00—Details of semiconductor or other solid state devices
    • H01L23/48—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor
    • H01L23/488—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor consisting of soldered or bonded constructions
    • H01L23/498—Leads, i.e. metallisations or lead-frames on insulating substrates, e.g. chip carriers
    • H01L23/49866—Leads, i.e. metallisations or lead-frames on insulating substrates, e.g. chip carriers characterised by the materials
    • H01L23/49877—Carbon, e.g. fullerenes
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C49/00—Alloys containing metallic or non-metallic fibres or filaments
    • C22C49/14—Alloys containing metallic or non-metallic fibres or filaments characterised by the fibres or filaments
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    • H01L23/00—Details of semiconductor or other solid state devices
    • H01L23/34—Arrangements for cooling, heating, ventilating or temperature compensation ; Temperature sensing arrangements
    • H01L23/36—Selection of materials, or shaping, to facilitate cooling or heating, e.g. heatsinks
    • H01L23/373—Cooling facilitated by selection of materials for the device or materials for thermal expansion adaptation, e.g. carbon
    • H—ELECTRICITY
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    • H01L23/00—Details of semiconductor or other solid state devices
    • H01L23/34—Arrangements for cooling, heating, ventilating or temperature compensation ; Temperature sensing arrangements
    • H01L23/36—Selection of materials, or shaping, to facilitate cooling or heating, e.g. heatsinks
    • H01L23/373—Cooling facilitated by selection of materials for the device or materials for thermal expansion adaptation, e.g. carbon
    • H01L23/3735—Laminates or multilayers, e.g. direct bond copper ceramic substrates
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10—Processes characterised by the sequence of their steps
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2999/00—Aspects linked to processes or compositions used in powder metallurgy
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C2204/00—End product comprising different layers, coatings or parts of cermet
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    • H01L2224/02—Bonding areas; Manufacturing methods related thereto
    • H01L2224/04—Structure, shape, material or disposition of the bonding areas prior to the connecting process
    • H01L2224/05—Structure, shape, material or disposition of the bonding areas prior to the connecting process of an individual bonding area
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    • H01L2224/42—Wire connectors; Manufacturing methods related thereto
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    • H01L2224/42—Wire connectors; Manufacturing methods related thereto
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    • H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
    • H01L2224/42—Wire connectors; Manufacturing methods related thereto
    • H01L2224/47—Structure, shape, material or disposition of the wire connectors after the connecting process
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    • H01L2224/481—Disposition
    • H01L2224/48151—Connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive
    • H01L2224/48221—Connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive the body and the item being stacked
    • H01L2224/48225—Connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive the body and the item being stacked the item being non-metallic, e.g. insulating substrate with or without metallisation
    • H01L2224/48227—Connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive the body and the item being stacked the item being non-metallic, e.g. insulating substrate with or without metallisation connecting the wire to a bond pad of the item
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    • H01L2224/80—Methods for connecting semiconductor or other solid state bodies using means for bonding being attached to, or being formed on, the surface to be connected
    • H01L2224/85—Methods for connecting semiconductor or other solid state bodies using means for bonding being attached to, or being formed on, the surface to be connected using a wire connector
    • H01L2224/8538—Bonding interfaces outside the semiconductor or solid-state body
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    • H01L24/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
    • H01L24/42—Wire connectors; Manufacturing methods related thereto
    • H01L24/47—Structure, shape, material or disposition of the wire connectors after the connecting process
    • H01L24/48—Structure, shape, material or disposition of the wire connectors after the connecting process of an individual wire connector
    • H—ELECTRICITY
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    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/0001—Technical content checked by a classifier
    • H01L2924/00014—Technical content checked by a classifier the subject-matter covered by the group, the symbol of which is combined with the symbol of this group, being disclosed without further technical details
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/01—Chemical elements
    • H01L2924/01019—Potassium [K]
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/01—Chemical elements
    • H01L2924/0102—Calcium [Ca]
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    • H01L2924/01079—Gold [Au]
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    • H01L2924/013—Alloys
    • H01L2924/0132—Binary Alloys
    • H01L2924/01322—Eutectic Alloys, i.e. obtained by a liquid transforming into two solid phases
    • H—ELECTRICITY
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    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/095—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00 with a principal constituent of the material being a combination of two or more materials provided in the groups H01L2924/013 - H01L2924/0715
    • H01L2924/097—Glass-ceramics, e.g. devitrified glass
    • H01L2924/09701—Low temperature co-fired ceramic [LTCC]
    • H—ELECTRICITY
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    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
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    • H01L2924/3025—Electromagnetic shielding
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K1/00—Printed circuits
    • H05K1/02—Details
    • H05K1/03—Use of materials for the substrate
    • H05K1/0306—Inorganic insulating substrates, e.g. ceramic, glass
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/0058—Laminating printed circuit boards onto other substrates, e.g. metallic substrates
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/38—Improvement of the adhesion between the insulating substrate and the metal
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/249921—Web or sheet containing structurally defined element or component
    • Y10T428/249924—Noninterengaged fiber-containing paper-free web or sheet which is not of specified porosity

Definitions

  • 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 .
  • 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.
  • 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.
  • 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.
  • metal-ceramic substrates as printed circuit boards, for example substrates made of aluminum oxide (Al 2 O 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.
  • 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.
  • the metals used especially in semiconductor modules or their substrates for the metallizations, the base plate, etc. must be able to conduct heat sufficiently.
  • materials with a copper or aluminum base such as Cu—W, Cu—Mo or Al—SiC are preferred especially for heat sinks.
  • 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.
  • 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.
  • This DCB method then comprises the following steps:
  • 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 .
  • 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.
  • 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.
  • the nanofibers are distributed isotropically in their orientation at least in the at least two spatial axes.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the metal and the nanofibers can be applied to a perform or a substrate made of metal and/or ceramic, for example through chemical and/or electrolytic precipitation.
  • HIP so-called HIP technology
  • 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.
  • the interior of the capsule is evacuated and sealed so that it is gastight.
  • 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° C.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the invention is used to create a composite material in which considerably higher conductivity (e.g. >380 W(mK) ⁇ 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.
  • 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.
  • 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) ⁇ 1 ) and their negligible thermal expansion coefficient, significantly reduce the expansion coefficient of the overall composite material and Improve its thermal conductivity.
  • FIG. 1 shows a simplified representation of an electric power module with a composite material according to the invention
  • 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
  • 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;
  • 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;
  • 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.
  • 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.
  • 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 .
  • 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.
  • 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.
  • 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 .
  • 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.
  • 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 .
  • 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.
  • 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.
  • 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.
  • 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 ⁇ 10 ⁇ 6 K ⁇ 1 , especially also in the relevant temperature range for substrates of semiconductor modules, i.e. between room temperature (approximately 20° C.) and 250° 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.
  • 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.
  • a coolant flows for example a gaseous and/or vaporous and/or liquid coolant, for example water.
  • 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.
  • 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.
  • an active cooler e.g. micro cooler
  • 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.
  • 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 .
  • 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.
  • a cap 10 is placed on the upper opening of the capsule 9 and is bonded tightly with the capsule, for example by welding.
  • 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.
  • the ductile, sealed capsule 9 is subjected to high pressure on all sides at a process temperature between 500 and 1,000° 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 .
  • 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.
  • the capsule 9 has a simple geometric shape and thin walls.
  • 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.
  • 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.
  • 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 .
  • FIGS. 4 and 5 show a further possibility for manufacturing the starter or row material, which contains the nanofibers in the metal matrix.
  • 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 .
  • 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.
  • 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.
  • 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.
  • 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 .
  • 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.
  • 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.
  • nanofibers or copper-coated nanofibers 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.
  • the power module 1 of FIG. 1 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.

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 12×10−6 K−1 in at least two axes of a three-dimensional system that are perpendicular in relation to each other.

Description

  • 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.
  • 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.
  • 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.
  • 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.
  • “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.
  • 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.
  • Composites with Carbon Fibrils™, 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.
  • 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.
  • 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.
  • 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.
  • Al2O3 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.
  • Due to the improved electrical properties, the use of metal-ceramic substrates as printed circuit boards, for example substrates made of aluminum oxide (Al2O3) 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.
  • 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.
  • 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
    Figure US20060263584A1-20061123-P00900
    of the material components of a power module with a copper-ceramic substrate are within the range of
    Figure US20060263584A1-20061123-P00900
    =16.8×10−6K−1 for copper and
    Figure US20060263584A1-20061123-P00900
    =3×10−6K−1 for silicon.
  • Reference is also made to the following table, in which the thermal conductivity
    Figure US20060263584A1-20061123-P00900
    and the thermal expansion coefficient
    Figure US20060263584A1-20061123-P00900
    are specified for various materials.
    □th in W/mK □ in 10−6/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
    Si3N4 10-40 3.1
    Al2O3 18.8 6.5
    FeNi42 15.1 5.1
    Silver epoxy cement 0.8-2   53
    Epoxide molding 0.63-0.76 18-30
    SiO2 0.1 0.5
    W 130 4.5
    Mo 140 5.1
    Cu/Mo/Cu 194 6.0
    AlSiC 160-220  7-10
  • 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.
  • 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.
  • This DCB method then comprises the following steps:
  • oxidation of a copper foil in a manner that results in an even copper oxide layer;
  • placing of the copper foil on the ceramic layer;
  • heating of this composite or strukture to a process temperature between approximately 1025 and 1083° C., e.g. to approximately 1071° C.;
  • cooling to room temperature.
  • 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.
  • 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.
  • 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.
  • In the embodiment of the composite material according to the invention, the following measures are possible in further embodiments of the invention:
  • The nanofibers are distributed isotropically in their orientation at least in the at least two spatial axes.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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° C.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • Therefore, the invention is used to create a composite material in which considerably higher conductivity (e.g. >380 W(mK)−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.
  • 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.
  • 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)−1) and their negligible thermal expansion coefficient, significantly reduce the expansion coefficient of the overall composite material and Improve its thermal conductivity.
  • The invention is described below in detail based on exemplary embodiments with reference to the drawings, wherein:
  • FIG. 1 shows a simplified representation of an electric power module with a composite material according to the invention;
  • 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;
  • 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;
  • 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;
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 <5×10−6K−1, especially also in the relevant temperature range for substrates of semiconductor modules, i.e. between room temperature (approximately 20° C.) and 250° 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.
  • The thermal conductivity
    Figure US20060263584A1-20061123-P00900
    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
    Figure US20060263584A1-20061123-P00900
    =600 W(mK)−1 or greater. Due to the extremely reduced thermal expansion coefficient
    Figure US20060263584A1-20061123-P00900
    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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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° 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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
    • 1 power module
    • 2 copper-ceramic substrate
    • 3 power component
    • 4 base plate
    • 5 ceramic layer
    • 6, 7 metallization
    • 8 mixture
    • 9 capsule
    • 10 cap
    • 11 cap connection
    • 12 chamber
    • 13 starting product of metal matrix with nanofibers
    • 14 rolling mechanism
    • 15 foil
    • 16 starter foil
    • 17 bath for co-precipitation of nanofibers and copper
    • 18 perform
    • 19 bath for precipitation of copper

Claims (34)

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 12×10−6K−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.
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