DE102023113513A1 - Metal-ceramic substrate and method for producing a metal-ceramic substrate - Google Patents

Abstract

Metal-ceramic substrate (1) as a carrier for electrical components, comprising:
- at least one metal layer (10) and
- a ceramic element (30), wherein the at least one metal layer (10) and the ceramic element (30) extend along a main extension plane (HSE) and are arranged one above the other along a stacking direction (S) running perpendicular to the main extension plane (HSE), wherein first grains (15) in the ceramic element (30) each have a maximum grain diameter D _max , a grain diameter D _Orth running perpendicular to D _max , determined at half the length of D _max , and a grain shape factor R = D _Orth / D _max , wherein the first (15) grains have an average grain shape factor, preferably determined as an arithmetic mean, which is less than 0.5, preferably less than 0.4 and particularly preferably less than 0.3 and wherein the first grains are aligned isotropically in the ceramic element.

Description

The present invention relates to a metal-ceramic substrate and a method for producing a metal-ceramic substrate.

Metal-ceramic substrates are well known as printed circuit boards or circuit boards from the state of the art, for example from DE 10 2013 104 739 A1 , the DE 19 927 046 B4 and the DE 10 2009 033 029 A1 Typically, connection surfaces for electrical components and conductor tracks are arranged on one component side of the metal-ceramic substrate or the metal-ceramic substrate, whereby the electrical components and the conductor tracks can be connected together to form electrical circuits. Essential components of the metal-ceramic substrates are an insulation layer, which is preferably made of a ceramic, and at least one metal layer connected to the insulation layer. Due to their comparatively high insulation strengths, insulation layers made of ceramic have proven to be particularly advantageous in power electronics. By structuring the metal layer, conductor tracks and/or connection surfaces for the electrical components can then be realized.

From the state of the art, it is known, for example, to produce ceramic elements, in particular aluminum nitride ceramic elements, in ceramic foil casting by using a so-called doctorblade process. This produces ceramic properties that, on the one hand, achieve a very high thermal conductivity with values between 170 W/mK and 230 W/mK, but, on the other hand, due to their structure, result in comparatively weak mechanical properties that reduce the service life of the ceramic element when used in power electronics substrates.

It is possible to improve these mechanical properties in aluminum nitride ceramic elements by doping them with zirconium dioxide, for example. However, this doping reduces the thermal performance of the ceramic element. In addition, zirconium dioxide doping only brings about a comparatively small improvement in fracture toughness of 10% to 15%.

Based on this, the present invention sets itself the task of providing metal-ceramic substrates whose ceramic elements have a comparatively high thermal conductivity and at the same time ensure a sufficiently high mechanical stability, which allows the metal-ceramic substrates to be used in power electronics.

The present invention solves this problem with a metal-ceramic substrate according to claim 1 and a method for producing a metal-ceramic substrate according to claim 8. Further embodiments can be found in the dependent claims and the description.

• - mindestens eine Metallschicht und
• - ein Keramikelement,

wobei sich die mindestens eine Metallschicht und das Keramikelement entlang einer Haupterstreckungsebene erstrecken und entlang einer senkrecht zur Haupterstreckungsebene verlaufenden Stapelrichtung übereinander angeordnet sind, wobei erste Körner im Keramikelement jeweils einen maximalen Korndurchmesser D_max, einen senkrecht zu D_max verlaufenen Korndurchmesser D_Orth, bestimmt auf halber Länge von D_max, und einen Korn-Formfaktor R= D_Orth / D_max aufweisen, wobei die ersten Körner einen mittleren Korn-Formfaktor, bestimmt als arithmetischer Mittelwert, aufweisen, der kleiner als 0,5, bevorzugt kleiner als 0,4 und besonders bevorzugt kleiner als 0,3 ist und wobei die ersten Körner isotrop im Keramikelement ausgerichtet sind.The present invention relates to a metal-ceramic substrate as a carrier for electrical components, in particular a metal-ceramic substrate usable or used as a circuit board, comprising:

• - at least one metal layer and
• - a ceramic element,

wherein the at least one metal layer and the ceramic element extend along a main extension plane and are arranged one above the other along a stacking direction running perpendicular to the main extension plane, wherein first grains in the ceramic element each have a maximum grain diameter D _max , a grain diameter D _Orth running perpendicular to D _max , determined at half the length of D _max , and a grain shape factor R = D _Orth / D _max , wherein the first grains have an average grain shape factor, determined as an arithmetic mean, which is less than 0.5, preferably less than 0.4 and particularly preferably less than 0.3 and wherein the first grains are isotropically aligned in the ceramic element.

Compared to the metal-ceramic substrates known from the prior art, the invention provides that the ceramic elements used here have first grains that are comparatively highly elliptical or rice grain-shaped. Such first grains, which are also referred to in particular as whiskers, are characterized by the fact that they can provide extremely high thermal conductivities. While it was usual in the prior art to align them in a targeted manner, the present invention provides for these first grains, which are highly elliptical in shape, to be aligned isotropically in the ceramic element, i.e. without them having a preferred direction. It has been found that, on the one hand, one can make use of the positive effect on thermal conductivity that comes from highly elliptical first grains, and, in addition, sufficiently high mechanical stability can be ensured if these first grains are distributed or aligned isotropically, i.e. evenly, in the ceramic element. In particular, the bending strength and cracking ability are improved compared to ceramic elements in which the first grains have a preferred direction. Basically, a reinforcement is achieved by the first grains, which acts like a fiber reinforcement in the ceramic structure. In this way, one can on the one hand, take advantage of the fact that high thermal conductivities can be provided and, on the other hand, a sufficiently high mechanical stability, which is particularly necessary when the metal-ceramic substrates are used as printed circuit boards and/or as a component of a printed circuit board.

The metal-ceramic substrate is preferably designed as a circuit board in which the at least one metal layer that is connected to the ceramic element is structured in the manufactured state. For example, it is provided that after the connection step, structuring is also carried out, for example by lasering, etching and/or mechanical processing, with which conductor tracks and/or connections for electrical or electronic components are realized. It is preferably provided that on a manufactured metal-ceramic substrate, on the ceramic element, on the side opposite the metal layer, a further metal layer, in particular a rear-side metallization and/or a cooling element, is provided. The rear-side metallization preferably serves to counteract deflection and the cooling element serves to effectively dissipate heat that emanates during operation from electrical or electronic components that are connected to the circuit board or the metal-ceramic substrate.

Copper, aluminum, molybdenum, tungsten, nickel and/or their alloys such as CuZr, AlSi or AlMgSi, as well as laminates such as CuW, CuMo, CuAl and/or AlCu or MMC (metal matrix composite), such as CuW, CuM or AlSiC, are conceivable as materials for the at least one metal layer and/or the at least one further metal layer in the metal-ceramic substrate or ceramic element. Furthermore, it is preferably provided that the at least one metal layer on the manufactured metal-ceramic substrate is surface-modified, in particular as component metallization. For example, a sealing with a precious metal, in particular silver and/or gold, or (electroless) nickel or ENIG (“electroless nickel immersion gold”) or edge casting on the metallization to suppress crack formation or widening is conceivable as a surface modification.

In particular, it is provided that the grains have an orientation direction that runs parallel to the direction along which the maximum grain diameter D _max is determined, wherein the orientation direction is inclined by an angle of inclination relative to the main extension plane, wherein the ceramic element has an average angle of inclination, determined as an arithmetic mean, to form an isotropic orientation, which assumes a value between -20° and 20°, preferably between -10° and 10° and particularly preferably between -5° and 5°. Second grains that have an essentially spherically symmetrical shape are not taken into account. The arithmetic mean, which assumes a value between -20° and 20°, preferably between -10° and 10° and particularly preferably between -5° and 5°, in particular reflects or quantifies an isotropic distribution of the orientation of the grains.

Furthermore, it is provided that the ceramic element is preferably assigned a grain shape factor frequency distribution with at least two maxima, with a first maximum being assigned to the first grains and a second maximum being assigned to second grains. In other words: the ceramic element comprises not only first grains that have a highly elliptical or rice grain-shaped shape, but also second grains that preferably have a substantially spherically symmetrical geometry. In order to assign a grain to the first grain or the second grain, it is advantageous to create a grain shape factor frequency distribution so that the grains that are assigned to the first maximum can be classified as first grains and the grains that are assigned to the second maximum can be classified as second grains. Within this frequency distribution, the shape factor of the first maximum can preferably be assumed to be the average grain shape factor, which assumes a value that is less than 0.5, preferably less than 0.4 and particularly preferably less than 0.3. The shape factor associated with the second maximum preferably has an average grain shape factor that is greater than 0.6, preferably greater than 0.7 and particularly preferably greater than 0.8. This expresses that, in addition to the elliptical or rice-shaped grains, spherically symmetrical grains or essentially spherically symmetrical grains are also present in the ceramic element.

In particular, it is possible to use the frequency distribution to determine that the positive properties that arise are essentially independent of the ratio of the first grains to the second grains. It is preferably provided that a ratio of the number of first grains to second grains in the sintered AIN body assumes a value that is less than 0.5, preferably less than 0.3 and preferably 0.2. It is preferably provided that a proportion of the first grains is set as low as necessary in order to achieve a desired increase in mechanical strength.

According to a particularly preferred embodiment, it is provided that a bonding layer is formed in the manufactured metal-ceramic substrate between the at least one metal layer and the ceramic element, wherein an adhesion-promoting layer of the bonding layer has a surface resistance that is greater than 5 Ohm/sq, preferably greater than 10 Ohm/sq and particularly preferably greater than 20 Ohm/sq.

The surface resistance is directly related to the proportion of active metal in the bonding layer, which is crucial for the connection of at least one metal layer to the ceramic element. The surface resistance increases with the proportion of active metal in the bonding layer. A correspondingly high surface resistance therefore corresponds to a low proportion of active metal in the bonding layer.

The surface resistance does not depend on a single parameter, but can be influenced by the interaction of several parameters. For example, the purity of the active metal, the thickness of the bonding layer and/or the surface roughness of the ceramic element also contribute to determining the surface resistance. In particular, high surface resistances can only be achieved through the interaction of at least two parameters.

It has been found that the formation of brittle, intermetallic phases is promoted with an increasing proportion of active metal, which in turn is detrimental to the peel strength of the metal layer on the insulation layer. In other words: the surface resistances as claimed describe bonding layers whose peel strength is improved, i.e. increased, due to the reduced formation of brittle intermetallic phases. By specifically setting the surface resistances as claimed, particularly strong bonds of the at least one metal layer to the ceramic element can be achieved. Such an increased bond strength has a beneficial effect on the service life of the metal-ceramic substrate. In order to determine the surface resistance, the metal layer and, if applicable, a solder base layer are first removed from the manufactured metal-ceramic substrate, for example by etching. A surface resistance is then measured using a four-point measurement on the outside or underside of the metal-ceramic substrate freed of the at least one metal layer and the solder base layer. In particular, the surface resistance of a material sample is to be understood as its resistance in relation to a square surface area. It is usual to indicate the surface resistance with the unit Ohm/sq(square). The physical unit of the surface resistance is Ohm. Preferably, it is provided that a thickness of the bonding layer measured in the stacking direction, averaged over several measuring points within a predetermined area or in several areas that run or run parallel to the main extension plane, assumes a value that is less than 0.20 mm, preferably less than 10 µm and particularly preferably less than 6 µm. When several areas are mentioned, this means in particular that the at least one metal layer is divided into areas that are as equal as possible and in each of these areas dividing the at least one metal layer, at least one value, preferably several measured values, are recorded for the thickness. The thicknesses thus determined at different points are arithmetically averaged.

Compared to the metal-ceramic substrates known from the prior art, a comparatively thin bonding layer is thus formed between the at least one metal layer and the ceramic element. In this case, it is provided that, in order to determine the relevant thickness of the bonding layer, the measured thicknesses are averaged over a large number of measuring points that lie within a predetermined or fixed area or the multiple areas. This advantageously takes into account the fact that the ceramic element is generally subject to undulation, i.e. the ceramic element is said to have a waviness. In particular, the person skilled in the art understands waviness to mean a modulation of the generally flat course of the ceramic element, seen over several millimeters or centimeters along a direction that runs parallel to the main extension plane. This distinguishes such an undulation from a surface roughness of the ceramic element, which is generally also present on the ceramic element. By including such a generally unavoidable undulation of the ceramic element in the determination of the thickness, it is taken into account that the bonding layer may vary due to the undulation, in particular it may be larger in valley areas of the ceramic element than in mountain areas of the ceramic element.

Preferably, a proportion of active metal in the adhesion promoter layer comprising an active metal is greater than 15% by weight, preferably greater than 20% by weight and particularly preferably greater than 25% by weight. It is also conceivable that the proportion of active metal in the adhesion promoter layer comprising an active metal is greater than 2% by weight, preferably greater than 3% by weight or preferably greater than 4% by weight or that the proportion of active metal in the adhesion promoter layer comprising an active metal is greater than 2% by weight, preferably greater than 5% by weight and particularly preferably greater than 10% by weight. This preferably depends on the bonding method selected.

It is preferably provided that a thermal conductivity of the ceramic element is greater than 130 W/mk, preferably greater than 140 W/mk and especially greater than 150 W/mk. It has been found that, in particular by a corresponding accumulation of first grains in the ceramic element, it is possible to realize comparatively high thermal conductivities, which have an advantageous effect on the thermal shock resistance of the manufactured metal-ceramic substrates, whereby thermomechanical stresses can be advantageously avoided, which could otherwise lead to failure and/or damage to the power module or to the metal-ceramic substrate as a circuit board.

It is particularly preferred that the ceramic element comprises aluminum nitride. In particular, it is conceivable that the first grains comprise aluminum nitride or consist of aluminum nitride. It is conceivable that the second grains also comprise aluminum nitride, so that the ceramic element preferably consists of aluminum nitride. Preferably, the ceramic element has Al ₂ O ₃ , Si ₃ N ₄ , AIN, an HPSX ceramic (ie a ceramic with an Al ₂ O ₃ matrix that comprises an x percent proportion of ZrO ₂ , for example Al ₂ O _S with 9% ZrO ₂ = HPS9 or Al ₂ O _S with 25% ZrO ₂ = HPS25), SiC, BeO, MgO, high-density MgO (> 90% of the theoretical density), TSZ (tetragonally stabilized zirconium oxide) as material for the ceramic. It is also conceivable that the ceramic element is designed as a composite or hybrid ceramic, in which several ceramic layers, each differing in terms of their material composition, are arranged one above the other and joined together to form a ceramic element in order to combine various desired properties.

Furthermore, it is preferably provided that the at least one metal layer and/or the at least one further metal layer is bonded to the ceramic element by means of an active soldering process and/or a hot isostatic pressing process and/or a DCB process.

• - Bereitstellen einer Lötschicht, insbesondere in Form mindestens einer Lötfolie bzw. Hartlotfolie,
• - Beschichten des Keramikelements und/oder der mindestens einen Metallschicht und/oder der mindestens einen Lötschicht mit mindestens einer Aktivmetallschicht,
• - Anordnen der mindestens einen Lötschicht zwischen dem Keramikelement und der mindestens einen Metallschicht entlang einer Stapelrichtung unter Ausbildung eines Lötsystems, das die mindestens eine Lötschicht und die mindestens eine Aktivmetallschicht umfasst, wobei ein Lotmaterial der mindestens einen Lötschicht vorzugsweise frei von einem schmelzpunkterniedrigenden Material bzw. von einem phosphorfreien Material ist, und
• - Anbinden der mindestens einen Metallschicht an die mindestens eine Keramikschicht über das Lötsystem mittels eines Aktivlotverfahrens.

For example, it is provided that a method for producing a metal-ceramic substrate is provided, comprising:

• - Providing a solder layer, in particular in the form of at least one solder foil or brazing foil,
• - coating the ceramic element and/or the at least one metal layer and/or the at least one solder layer with at least one active metal layer,
• - arranging the at least one solder layer between the ceramic element and the at least one metal layer along a stacking direction to form a solder system which comprises the at least one solder layer and the at least one active metal layer, wherein a solder material of the at least one solder layer is preferably free of a melting point-lowering material or of a phosphorus-free material, and
• - Bonding the at least one metal layer to the at least one ceramic layer via the soldering system by means of an active soldering process.

In particular, a multi-layer soldering system is provided, comprising at least one soldering layer, preferably free of melting point-lowering elements, particularly preferably a phosphorus-free soldering layer, and at least one active metal layer. The separation of the at least one active metal layer and the at least one soldering layer proves to be particularly advantageous because it enables comparatively thin soldering layers to be realized, particularly when the soldering layer is a foil. For soldering materials containing active metals, comparatively large soldering layer thicknesses must otherwise be realized due to the brittle intermetallic phases or the high modulus of elasticity and high yield strength of the common active metals and their intermetallic phases, which hinder the deformation of the soldering paste or soldering layer, whereby the minimum layer thickness is limited by the manufacturing properties of the soldering material containing active metal. Accordingly, for solder layers containing active metals, it is not the minimum thickness required for the joining process that determines the minimum solder layer thickness of the solder layer, but the minimum layer thickness of the solder layer that is technically feasible that determines the minimum solder layer thickness of the solder layer. This makes this thicker, active metal-containing solder layer more expensive than thin layers. The expert understands phosphorus-free in particular to mean that the proportion of phosphorus in the solder layer is less than 150 ppm, less than 100 ppm and particularly preferably less than 50 ppm.

In particular, by using a separately implemented active metal layer, it is possible to make it comparatively thin, whereby the required comparatively thin thicknesses of the bonding layer can be achieved, in particular averaged over different measurement values within the specified area or areas. Examples of an active metal are titanium (Ti), zirconium (Zr), hafnium (Hf), chromium (Cr), niobium (Nb), cerium (Ce), tantalum (Ta), magnesium (Mg), lanthanum (La) and vanadium (V). It should be noted that the metals La, Ce, Ca and Mg can easily oxidize. It is also noted that the elements Cr, Mo and W are not classic active metals, but are suitable as a contact layer between Si ₃ N ₄ and the at least one metal layer or the solder system or solder material, since they are compatible with the at least one metal layer, for example copper, does not form intermetallic phases and does not have edge solubility.

Preferably, the solder layer, in particular the phosphorus-free solder layer, comprises several materials in addition to the pure metal. For example, indium is a component of the solder material used in the solder layer.

Furthermore, it is conceivable that the solder material for forming the solder layer is applied to the active metal layer and/or the at least one metal layer by physical and/or chemical vapor deposition and/or galvanically. This advantageously makes it possible to realize comparatively thin solder layers in the soldering system, in particular in a homogeneous distribution.

• - Bereitstellen eines Keramikelements und einer Metalllage,
• - Bereitstellen eines gasdichten Behälters, der das Keramikelement umschließt, wobei der Behälter vorzugsweise aus der Metalllage geformt ist oder die Metalllage umfasst,
• - Ausbilden des Metall-Keramik-Substrats durch ein Anbinden der Metalllage an das Keramikelement mittels heißisostatischem Pressen,

wobei zum Ausbilden des Metall-Keramik-Substrats zwischen der Metalllage und dem Keramikelement mindestens abschnittsweise eine Aktivmetallschicht oder eine ein Aktivmetall umfassende Kontaktschicht, zur Unterstützung des Anbindens der Metalllage an das Keramikelement, angeordnet wird. Der Behälter wird dabei vorzugsweise als Metallbehälter aus einer Metalllage und/oder einer weiteren Metalllage gebildet. Alternativ ist es auch vorstellbar, dass ein Glasbehälter verwendet wird..For example, in the production of the metal-ceramic substrate, in particular the metal-ceramic substrate, further steps are provided, comprising:

• - Providing a ceramic element and a metal layer,
• - providing a gas-tight container enclosing the ceramic element, wherein the container is preferably formed from the metal layer or comprises the metal layer,
• - Forming the metal-ceramic substrate by bonding the metal layer to the ceramic element by means of hot isostatic pressing,

wherein, to form the metal-ceramic substrate, an active metal layer or a contact layer comprising an active metal is arranged between the metal layer and the ceramic element at least in sections to support the bonding of the metal layer to the ceramic element. The container is preferably formed as a metal container made of a metal layer and/or a further metal layer. Alternatively, it is also conceivable that a glass container is used.

In hot isostatic pressing, it is particularly intended that the bonding takes place by heating under pressure, in which the first and/or second metal layer of the metal container, in particular the subsequent metal layer of the metal-ceramic substrate and any eutectic layer occurring there, do not enter the melting phase. Accordingly, lower temperatures are required for hot isostatic pressing than for a direct metal bonding process, in particular a DCB process.

In comparison to the connection of a metal layer to a ceramic layer by means of a solder material, in which temperatures below the melting temperature of the at least one metal layer are usually used, the present procedure advantageously makes it possible to dispense with a solder base material and only requires an active metal. The use or utilization of pressure during hot isostatic pressing also proves to be advantageous because it can reduce air inclusions or cavities between the first metal layer and/or the second metal layer on the one hand and the ceramic element on the other, whereby the frequency of the formation of cavities in the formed or manufactured metal-ceramic substrate can be reduced or even avoided. This has a beneficial effect on the quality of the bond between the metal layer or the first and/or second metal layer of the metal container and the ceramic element. In addition, it is advantageously possible to simplify the "second etching" and avoid solder residues and silver migration.

It is also conceivable that during hot isostatic pressing an additional solder material is introduced between the ceramic element and the at least one metal layer, wherein a melting temperature of the additional solder material can be lower than the temperature at which the hot isostatic pressing is carried out, i.e. lower than the melting temperature of the at least one metal layer.

It is preferably provided that during hot isostatic pressing of the metal container in a heating and pressure device is exposed to a gas pressure of between 100 and 2000 bar, preferably between 150 and 1200 bar and particularly preferably between 300 and 1000 bar and a process temperature of 300 °C up to a melting temperature of the at least one metal layer, in particular up to a temperature below the melting temperature. It has advantageously been found that it is thus possible to bond a metal layer, i.e. a first and/or second metal layer of the metal container, to the ceramic element without the required temperatures of a direct metal bonding process, for example a DCB or a DAB process, and/or without a solder base material that is used in active soldering. In addition, the use or application of an appropriate gas pressure makes it possible to produce a metal-ceramic substrate that is as free of cavities as possible, i.e. without gas inclusions between the metal layer and the ceramic element. In particular, process parameters are used which are DE 2013 113 734 A1 mentioned and to which explicit reference is hereby made.

Furthermore, it is particularly preferred that the ceramic element, in particular the first grains and/or second grains, are monocrystalline. This has a particularly advantageous effect on the thermal conductivity of the metal-ceramic substrate.

• - Bereitstellen eines Granulats, das erste Körner mit einem ersten Kornformfaktor umfasst, der im arithmetischen Mittel kleiner ist als 0,5, bevorzugt kleiner als 0,4 und besonders bevorzugt kleiner als 0,3, und zweite Körnern mit einem zweiten Kornformfaktor umfasst, der im arithmetischen Mittel größer ist als 0,5, bevorzugt größer als 0,6 und besonders bevorzugt größer ist als 0,7,
• - Pressen, insbesondere isotropes Pressen des Granulats in eine Blockform und
• - Sintern des blockförmigen Granulats zur Bildung eines Keramikblocks.

A further aspect of the present invention is a method for producing a ceramic element and/or a metal-ceramic substrate according to the invention. It is provided that the following steps are provided for providing the ceramic element:

• - Providing a granulate comprising first grains with a first grain shape factor which is on average less than 0.5, preferably less than 0.4 and particularly preferably less than 0.3, and second grains with a second grain shape factor which is on average greater than 0.5, preferably greater than 0.6 and particularly preferably greater than 0.7,
• - Pressing, in particular isotropic pressing of the granulate into a block shape and
• - Sintering the block-shaped granules to form a ceramic block.

All advantages described for the metal-ceramic substrate also apply to the process for producing the ceramic element and/or the metal-ceramic substrate according to the invention and vice versa.

In particular, it is provided that the pressing of the granulate takes place under the influence of pressure, wherein the pressure preferably assumes a value between 50 bar and 2000 bar, preferably between 100 and 1500 bar and particularly preferably 150 and 1000 bar. The lower limit is preferably determined by uniaxial pressing and the upper limit by cold isostatic pressing. It is also conceivable that the sintering takes place at a temperature that assumes a value of 1600 °C and 2000 °C, preferably between 1700 °C and 1950 °C and particularly preferably between 1750 °C and 1900 °C. It is also preferably provided that pressing and sintering are carried out in successive steps and/or at least partially overlapping in time. In this way, it is possible to produce a ceramic element in which first grains are formed in the ceramic element that are isotropic, i.e. evenly aligned.

Preferably, a ceramic element is separated from the ceramic block by means of a sawing element, in particular by means of a diamond wire saw. This advantageously makes it possible to provide ceramic elements from the ceramic block. These separated ceramic elements can then be used as ceramic elements for a large card by bonding a metal layer or at least one metal layer to the ceramic element to form a metal-ceramic substrate that can be used as a circuit board after the metal layer has been structured. It is advantageous to separate the ceramic element from the block because this allows, for example, the choice of thickness for the ceramic element to be flexibly adjusted. In contrast, it is standardized in the prior art to sinter ceramic elements with a predetermined thickness.

Further advantages are the surface properties on the top and back of the ceramic element, which correspond to one another when several ceramic elements are cut out of a ceramic block. This does not occur when using the conventional production of ceramic elements. It is also conceivable to produce comparatively thin ceramic elements, in particular ceramic elements that only have a thickness of 100 µm or less. It is also possible to produce substrates that have a thickness that is significantly greater than 1 mm. It is also possible to use a diamond sawing technique that is already established in the silicon wafer industry.

Preferably, it is provided that at least one metal layer is bonded to the ceramic element and preferably structured to form conductor tracks. This makes it advantageously possible to use the ceramic element produced with the properties described as a metal-ceramic substrate that is used in power electronics, in particular as a circuit board in which individual metal sections are insulated from one another in a component metallization for the purpose of forming conductor tracks and connection surfaces.

• Weitere Vorteile und Eigenschaften ergeben sich aus der nachfolgenden Beschreibung bevorzugter Ausführungsformen des erfindungsgemäßen Gegenstands mit Bezug auf die beigefügten Figuren. Es zeigen:
  • 1: Metall-Keramik-Substrat gemäß einer beispielhaften Ausführungsform der vorliegenden Erfindung;
  • 2 eine erste Körnung aus einer Vielzahl von ersten Körnern für das Keramikelement an dessen Außenseite
  • 3a-3d ein Verfahren zur Herstellung eines Keramikelements

Further advantages and properties emerge from the following description of preferred embodiments of the subject matter according to the invention with reference to the attached figures. They show:

• Further advantages and properties emerge from the following description of preferred embodiments of the subject matter according to the invention with reference to the attached figures. They show:
  • 1 : Metal-ceramic substrate according to an exemplary embodiment of the present invention;
  • 2 a first grain of a plurality of first grains for the ceramic element on its outside
  • 3a-3d a method for producing a ceramic element

In the 1 a metal-ceramic substrate 1 is shown according to a first exemplary embodiment of the present invention. Such metal-ceramic substrates 1 preferably serve as a carrier or circuit board for electronic or electrical components that can be connected to the at least one metal layer 10 of the metal-ceramic substrate 1 on its component side. It is preferably provided that the at least one metal layer 10 is structured in order to form corresponding conductor tracks and/or connection surfaces, ie in the manufactured metal-ceramic substrate 1 the at least one metal layer 10 comprises several metal sections that are electrically insulated from one another. The at least one metal layer 10 extending essentially along a main extension plane HSE and a ceramic element 30 extending along the main extension plane HSE are arranged one above the other along a stacking direction S running perpendicular to the main extension plane HSE and are preferably joined or connected to one another via a bonding layer 12. Preferably, the metal-ceramic substrate 1 comprises, in addition to the at least one metal layer 10, at least one further metal layer 20, which, as seen in the stacking direction S, is arranged on the side of the ceramic element 30 opposite the at least one metal layer 10 and is bonded to the ceramic element 30 via a further bonding layer 12'.

The at least one further metal layer 20 serves as a rear-side metallization, which counteracts a bending of the metal-ceramic substrate 1, in particular of the metal-ceramic element 1, and/or as a heat sink, which is designed to dissipate heat input caused by electrical or electronic components on the metal-ceramic substrate 1.

In particular, the metal-ceramic substrate 1 has a bonding layer 12 arranged between the at least one metal layer 10 and the ceramic element 30. It has proven to be advantageous if a thickness of the bonding layer 12 measured in the stacking direction S is comparatively thin. In addition, a comparatively thin thickness of the bonding layer 12 between the at least one metal layer 10 and the ceramic element 30 proves to be advantageous if an etching process is provided for the purpose of structuring the at least one metal layer 10. For example, narrower isolation trenches, i.e. distances between individual metal sections of the at least one metal layer 10, can be realized in this way.

Furthermore, the formation of a thinner bonding layer 12 proves to be advantageous in that it can further reduce the number of possible defects in the bonding layer 12 caused by material defects in a solder material that may be used.

In the 1 In the example shown, the bonding layer 12 is in particular an adhesion promoter layer 13 comprising an active metal. In this case, the adhesion promoter layer 13 is preferably formed after bonding from a material composition which comprises a compound of components of the ceramic element on the one hand and an active metal on the other. Since these are very brittle compounds, it is advantageous for this adhesion promoter layer 13 to be as thin as possible for the adhesive strength of the at least one metal layer 10 on the ceramic element 30. For example, the adhesion promoter layer 13 can form the bonding layer 12 if, for example, an active metal layer, in particular an active metal foil, is arranged for the bonding process between the ceramic element 30 and the metal layer 10 and the bonding process takes place via hot isostatic pressing. However, the adhesion promoter layer 13 can also be formed, for example, by an active metal layer, in particular an active metal foil, which is arranged between the ceramic element 30 and a solder base layer in order to create the bond between the metal layer 10 and the ceramic element 30 via the system of active metal layer and solder base layer. In this case, the adhesion promoter layer 13 forms part of the bonding layer 12. In particular, the active metal layer comprises a proportion of an active metal that is greater than 15% by weight, preferably greater than 50% by weight and particularly preferably greater than 75% by weight.

In 2 a first grain size of a plurality of first grains 15 for the ceramic element 30 is shown on its outside, in particular on the outside facing the metal layer 10 in the metal-ceramic substrate 1. This is the first grain size in a sectional view through the metal-ceramic substrate 1 along a plane running perpendicular to the main extension plane HSE. The grain sizes shown here are characterized by their particularly elliptical or elongated shape. It is provided that the first grains 15 of this first grain size in the ceramic element 30 each have a maximum grain diameter D _max and a grain diameter D _Orth running perpendicular to D _max . The grain diameter running perpendicular to D _max The diameter is determined at half the length of D _max , so that it is possible to determine a form factor for each of the grains according to the relationship R = D _Orth / D _max . Preferably, the grains are measured on the surface or outside or on a cutting plane of the ceramic element, which can be seen in a SEM image in a section through the metal-ceramic substrate 1.

The ceramic element 30, in particular the first grains 15 on the outside of the ceramic element 30, is thus assigned an average shape factor which is determined as an arithmetic mean, preferably from at least 100 grains 15, which in turn is less than 0.5, preferably less than 0.4 and particularly preferably less than 0.3. This describes in particular those first grains 15 which have a substantially strongly elliptical or non-circular shape. The grains 15 are drawn elongated and have a main direction of extension. For example, the grains 15 are shaped like grains of rice.

In particular, it is provided that in the ceramic element 30 the first grains 15, whose average form factor is less than 0.5, preferably less than 0.4 and particularly preferably less than 0.3, are aligned isotropically. In other words: the highly elliptical first grains 15 introduced into the ceramic element 30 have no preferred direction and are evenly distributed in all spatial directions. It has been found that a corresponding isotropic alignment of the grains within the ceramic element 30 leads to a reinforcement in the structure. At the same time, the highly elliptical first grains 15 support the formation of a comparatively high thermal conductivity, preferably of values greater than 120 W/mk. Finally, it has also been shown that the isotropic alignment of the individual first grains 15 leads to an increase in the bending strength, in particular because the isotropically aligned elliptical first grains 15 act like a fiber reinforcement in the ceramic structure of the ceramic element 30.

Preferably, in addition to the first grains 15, which have a comparatively small shape factor and are thus highly elliptical, the ceramic element 30 comprises second grains, the second grains being assigned a shape factor or an average grain shape factor which, as an arithmetic mean, assumes a value that is greater than 0.6, preferably greater than 0.7 and particularly preferably greater than 0.8. The second grains are therefore those that have a substantially spherical circumference. To classify the grains into first grains or second grains, the respective grain shape factors are determined and represented in a grain shape factor frequency distribution. This grain shape factor distribution has two maxima, with an accumulation of the shape factors in a first maximum at a value below 0.5 and a second maximum for the grain shape factors that have a value that is greater than 0.5. The first grains 15 are assigned to the first maximum and the second grains to the second maximum. In other words: the ceramic element 30 has grains that have a strongly rice grain-like shape and those that have a substantially spherical shape. By dividing it into first grains 15 and second grains, instead of describing an average grain factor that results across all grains, it is thus possible to discuss the isotropic alignment in connection with the strongly elliptical grains or first grains. The description is therefore essentially independent of the respective relative proportions of the first grains and second grains in the ceramic element. If one were to consider the shape factor across all grains, the average shape factor would change its value due to the presence of the second grains and in particular determined by the ratio of first grains and second grains in terms of their number.

Preferably, the number of second grains and first grains is substantially equal. Preferably, a ratio of the number of first grains to second grains in the sintered AlN body assumes a value that is less than 0.5, preferably less than 0.3 and preferably 0.2.

In the 2 In the cross-sectional image shown, only the first grains 15 are shown in order to determine a definition for the grain diameters.

In the 3a-3d a method for producing a ceramic element 30 is shown. In particular, the embodiment shown here provides that in a first step a granulate is produced by providing a powder, in particular an AIN powder with first grains 15 or starting grains that have a form factor that is less than 0.5, preferably less than 0.4 and particularly preferably less than 0.3. Such starting grains are also referred to as whiskers, in particular as AIN whiskers, and are typically produced as part of a doctorblade process.

Whiskers or AIN are preferably mixed with sintering aids such as Y ₂ O ₃ or ZrO ₂ and solvents (e.g. ethanol) and binding agents. These should not be crushed, but powder clusters should be broken down. The resulting slurry is converted into granules in a spray tower.

In 3b the second process step is shown. Here, the granulate produced in the first process step is pressed into a block. A pressure of between 50 bar and 2000 bar, preferably between 100 and 1500 bar and particularly preferably between 150 and 1000 bar is used. In particular, a block is created which is at least 30 mm, preferably at least 50 mm and particularly preferably at least 100 mm in size on its shortest side and at least 230 mm, preferably at least 250 mm and particularly preferably at least 270 mm in size on its longest side. The block preferably has a rectangular cross-section and is cylindrical. The block preferably has a volume of between 1.0 dm ³ and 10 dm ³ , preferably between 1.5 dm ³ and 5 dm ³ or 2 dm ³ and 8 dm ³ .

In the 3c The sintering step is shown, ie the third process step in which a ceramic block is created by applying pressure and temperature. The granulate now becomes a solid body.

In order to use the ceramic material produced in this way so that ceramic elements 30 can be used for the formation of metal-ceramic substrates, the ceramic block is divided into individual disc-shaped ceramic elements 30. For this purpose, disc-shaped ceramic elements 30 are cut out of the ceramic block using a sawing element, which in particular have the dimensions of a large card. The production using a corresponding block ceramic element and the subsequent division with a sawing element, in particular with a diamond-encompassing wire saw, makes it possible to produce ceramic elements with an adjustable thickness. This means that the thickness of the ceramic element 30 provided in production can be selected flexibly.

List of reference symbols:

1

metal-ceramic substrate

10

metal layer

12

binding layer

12'

additional binding layer

13

adhesion promoter layer

15

first grains

20

additional metal layer

30

ceramic layer

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• DE 10 2013 104 739 A1 [0002]
• DE 19 927 046 B4 [0002]
• DE 10 2009 033 029 A1 [0002]
• DE 2013 113 734 A1 [0032]

Claims (10)

Metal-ceramic substrate (1) as a carrier for electrical components, which serves in particular as a circuit board for electrical components, comprising: - at least one metal layer (10) and - a ceramic element (30), wherein the at least one metal layer (10) and the ceramic element (30) extend along a main extension plane (HSE) and are arranged one above the other along a stacking direction (S) running perpendicular to the main extension plane (HSE), wherein first grains (15) in the ceramic element (30) each have a maximum grain diameter D _max , a grain diameter D _Orth running perpendicular to D _max , determined at half the length of D _max , and a grain shape factor R = D _Orth / D _max , wherein the first grains (15) have an average grain shape factor, preferably determined as an arithmetic mean, which is less than 0.5, preferably less than 0.4 and particularly preferably less than 0.3 and wherein the first grains (15) isotropically aligned in the ceramic element (30). Metal-ceramic substrate (1) according to claim 1 , wherein the ceramic element (30) has a grain form factor frequency distribution with at least two maxima, wherein a first maximum is associated with the first grains (15) and a second maximum is associated with second grains. Metal-ceramic substrate (1) according to claim 1 or 2 , wherein in the manufactured metal-ceramic substrate (1) a bonding layer (12) is formed between the at least one metal layer (10) and the ceramic element (30), wherein an adhesion promoter layer (13) of the bonding layer (12) has a surface resistance which is greater than 5 Ohm/sq, preferably greater than 10 Ohm/sq and particularly preferably greater than 20 Ohm/sq. Metal-ceramic substrate (1) according to one of the preceding claims, wherein a thermal conductivity of the ceramic element (30) is greater than 130 W/mK, preferably greater than 140 W/mK and particularly preferably greater than 150 W/mK. Metal-ceramic substrate (1) according to one of the preceding claims, wherein the ceramic element (30) comprises aluminum nitride. Metal-ceramic substrate (1) according to one of the preceding claims, wherein the at least one metal layer (10) is bonded to the ceramic element (30) using a direct bonding method, an (active) soldering method, a diffusion bonding method and/or hot isostatic pressing. Metal-ceramic substrate (1) according to one of the preceding claims, wherein the first grains (15) are single-crystalline. Method for producing a ceramic element (30) and/or a metal-ceramic substrate (1) according to one of the preceding claims, wherein the provision of the ceramic element (30) comprises the following steps: - providing a granulate comprising first grains (15) with a first grain shape factor which is on average less than 0.5, preferably less than 0.4 and particularly preferably less than 0.25 and second grains with a second grain shape factor which is on average greater than 0.6, preferably greater than 0.7 and particularly preferably greater than 0.8, - pressing, in particular isotropic pressing, the granulate into a block shape and - sintering the block-shaped granulate to form a ceramic block. procedure according to claim 8 , wherein a ceramic element (30) is separated from the ceramic block by means of a sawing element, in particular by means of a diamond wire saw. Method according to one of the preceding claims, wherein at least one metal layer (10) is bonded to the ceramic element (30) and is preferably structured to form conductor tracks.

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