Metal-ceramic substrate and process for producing a metal-ceramic substrate

Abstract

A metal-ceramic substrate (1) as base for electrical components, comprising • - at least one metal layer (10) and • - a ceramic element (30), where the at least one metal layer (10) and the ceramic element (30) extend along a plane of principal extent (HSE) and are arranged one above another in a stacking direction (S) running perpendicularly to the plane of principal extent (HSE), where first grains (15) in the ceramic element (30) have respectively a maximum grain diameter Dmax, a grain diameter Dörth running perpendicularly to Dmax and determined at half the length of Dmax, and a grain form factor R= Dörth / Dmax, where the first grains (15) have a mean grain form factor, preferably determined as the arithmetic mean value, of less than 0.5, preferably less than 0.4 and more preferably less than 0.3, and where the first grains are oriented isotropically in the ceramic element.

Description

[0001] Metal-ceramic substrate and method for producing a metal-ceramic substrate

[0002] The present invention relates to a metal-ceramic substrate and a method for producing a metal-ceramic substrate.

[0003] Metal-ceramic substrates are well known as printed circuit boards or circuit boards from the prior art, for example from DE 10 2013 104 739 A1, DE 19 927 046 B4, and DE 10 2009 033 029 A1. Typically, connection pads for electrical components and conductor tracks are arranged on one component side of the metal-ceramic substrate or the metal-ceramic substrate, wherein the electrical components and the conductor tracks can be interconnected to form electrical circuits. Key components of the metal-ceramic substrates are an insulating layer, preferably made of a ceramic, and at least one metal layer bonded to the insulating layer. Due to their comparatively high insulation strengths, insulating layers made of ceramic have proven particularly advantageous in power electronics.By structuring the metal layer, conductor tracks and / or connection surfaces for the electrical components can then be realized.

[0004] For example, it is known from the prior art to produce ceramic elements, particularly aluminum nitride ceramic elements, by ceramic tape casting using a so-called doctorblade process. This produces ceramic properties that, on the one hand, achieve very high thermal conductivity with values ​​between 170 W / mK and 230 W / mK, but, on the other hand, result in comparatively weak mechanical properties due to their structure, which reduces the service life of the ceramic element when used in power electronics substrates.

[0005] 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. Furthermore, zirconium dioxide doping only results in a comparatively small improvement in fracture toughness of 10% to 15%.

[0006] 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.

[0007] 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 11. Further embodiments can be found in the dependent claims and the description.

[0008] 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 printed circuit board, comprising:

[0009] - at least one metal layer and

[0010] - 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 ma x, one perpendicular to D max Dörth grain diameter, determined at half the length of D ma x, and a grain shape factor R= Dörth / D ma x, 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 aligned isotropically in the ceramic element.

[0011] 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 common in the prior art to align them in a targeted manner, the present invention specifically 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, the positive effect on thermal conductivity that emanates from highly elliptically shaped first grains can be utilized, and, in addition, sufficiently high mechanical stability can be ensured if these first grains are isotropic, i.e.,are evenly distributed or aligned within the ceramic element. In particular, the flexural strength and crack resistance are improved compared to ceramic elements in which the first grains exhibit a preferred direction. Essentially, the first grains provide reinforcement, acting like fiber reinforcement in the ceramic structure. This approach allows for the provision of high thermal conductivities and, at the same time, sufficiently high mechanical stability, which is particularly necessary when the metal-ceramic substrates are used as printed circuit boards and / or as components of a printed circuit board.

[0012] The metal-ceramic substrate is preferably designed as a printed circuit board in which, in the manufactured state, the at least one metal layer which is bonded to the ceramic element is structured. For example, it is provided for this purpose that after the bonding 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 created. 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 back-side metallization and / or a cooling element, is provided. The back-side metallization preferably serves to counteract bending and the cooling element serves to effectively dissipate heat which is generated during operation of electrical or electronic components.electronic components that are connected to the circuit board or the metal-ceramic substrate.

[0013] Conceivable 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 include 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. 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. A possible surface modification could, for example, be a sealing with a precious metal, in particular silver and / or gold, or (electroless) nickel or EN IG ("electroless nickel immersion gold"), or edge encapsulation on the metallization to suppress crack formation or crack widening.

[0014] In particular, it is intended that the grains have an orientation direction that runs parallel to the direction along which the maximum grain diameter D ma x, 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 alignment, which assumes a value between -20° and 20°, preferably between -10° and 10° and particularly preferably between -5° and 5°. Second grains which have a substantially 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.

[0015] Furthermore, it is provided that the ceramic element is preferably assigned a grain shape factor frequency distribution with at least two maxima, wherein a first maximum is assigned to the first grains and a second maximum is assigned to second grains. In other words, the ceramic element comprises not only first grains that have a highly elliptical or rice-grain-shaped configuration, but also second grains that preferably have a substantially spherically symmetrical geometry. In order to assign a grain to the first grain size or the second grain size, it is advantageous to create a grain shape factor frequency distribution such that the grains assigned to the first maximum can be classified as first grains and the grains 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.

[0016] In particular, using the frequency distribution, it is possible to determine that the resulting positive properties are essentially independent of the quantitative ratio of first grains to second grains. Preferably, the 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 less than 0.2. Preferably, the proportion of first grains is set as low as necessary to achieve a desired increase in mechanical strength.

[0017] According to a particularly preferred embodiment, it is provided that a bonding layer is formed between the at least one metal layer and the ceramic element in the manufactured metal-ceramic substrate, wherein an adhesion-promoting layer of the bonding layer has a surface resistance which is greater than 5 ohms / sq, preferably greater than 10 ohms / sq and particularly preferably greater than 20 ohms / sq.

[0018] The sheet resistance is directly related to the active metal content of the bonding layer, which is crucial for bonding the at least one metal layer to the ceramic element. The sheet resistance increases with the active metal content in the bonding layer. A correspondingly high sheet resistance thus corresponds to a low active metal content in the bonding layer.

[0019] The sheet 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 sheet resistance. In particular, high sheet resistances can only be achieved through the interaction of at least two parameters.

[0020] It has been found that with an increasing proportion of active metal, the formation of brittle, intermetallic phases is promoted, which in turn is detrimental to the peel strength of the metal layer on the insulation layer. In other words: the claimed sheet resistances describe bonding layers whose peel strength is improved, i.e. increased, due to the reduced formation of brittle intermetallic phases. By specifically adjusting the claimed sheet resistances, particularly strong bonds between the at least one metal layer and the ceramic element can be achieved. Such an increased bond strength has a beneficial effect on the service life of the metal-ceramic substrate. To determine the sheet resistance, it is provided that the metal layer and, if applicable,a solder base layer is removed again, for example by etching. Using a four-point measurement, a sheet resistance is then measured on the outside or underside of the metal-ceramic substrate freed from the at least one metal layer and the solder base layer. In particular, the sheet resistance of a material sample is to be understood as its resistance relative to a square surface area. It is customary to designate the surface resistance with the unit ohm / sq(square). The physical unit of sheet 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 pm and particularly preferably less than 6 pm.When referring to multiple areas, this specifically means that the at least one metal layer is divided into areas of as equal size as possible, and at least one thickness value, preferably several measured values, are recorded for each of these areas dividing the at least one metal layer. The thicknesses thus determined at different locations are arithmetically averaged.

[0021] 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 which lie within a predetermined or fixed area or areas. This advantageously takes into account the fact that the ceramic element is generally subject to undulation, i.e. the ceramic element is to be attributed a waviness. In particular, the person skilled in the art understands waviness to be a modulation of the generally flat course of the ceramic element, viewed over several millimeters or centimeters along a direction which runs parallel to the main plane of extension.This distinguishes such undulation from surface roughness of the ceramic element, which is usually also present on the ceramic element. By including such, generally unavoidable undulation of the ceramic element in the thickness determination, it is taken into account that the bonding layer may vary due to the undulation, in particular, it may be larger in the valley areas of the ceramic element than in the peak areas.

[0022] Preferably, the proportion of active metal in the adhesion promoter layer comprising an active metal is greater than 15 wt. %, preferably greater than 20 wt. %, and particularly preferably greater than 25 wt. %. It is also conceivable for the proportion of active metal in the adhesion promoter layer comprising an active metal to be greater than 2 wt. %, preferably greater than 3 wt. % or preferably greater than 4 wt. %, or for the proportion of active metal in the adhesion promoter layer comprising an active metal to be greater than 2 wt. %, preferably greater than 5 wt. % and particularly preferably greater than 10 wt. %. This preferably depends on the bonding method chosen.

[0023] Preferably, the thermal conductivity of the ceramic element is greater than 130 W / mk, preferably greater than 140 W / mk, and particularly greater than 150 W / mk. It has been found that, in particular, by appropriately accumulating the first grains in the ceramic element, it is possible to achieve comparatively high thermal conductivities, which have a beneficial effect on the thermal shock resistance of the manufactured metal-ceramic substrates, thereby advantageously avoiding thermomechanical stresses that could otherwise lead to failure and / or damage to the power module or to the metal-ceramic substrate as a printed circuit board.

[0024] It is particularly preferably provided that the ceramic element comprises aluminum nitride (AIN). 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 Al2O3, SiSn4, AIN, an HPSX ceramic (i.e. a ceramic with an Al2O3 matrix which comprises an x ​​percent proportion of ZrO2, for example Al2O3 with 9% ZrO2 = HPS9 or Al2O3 with 25% ZrO2 = 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 orHybrid ceramic is formed in which, in order to combine various desired properties, several ceramic layers, each differing in terms of their material composition, are arranged one above the other and combined to form a ceramic element.

[0025] In particular, it is provided that the first grains and the second grains have the same ceramic material composition. For example, the first grains and the second grains are both formed from AlN. Another ceramic material forming the first and second grains is also conceivable. Furthermore, it is preferably provided that the ceramic element, in particular the first grains and / or the second grains, comprises less than 50 wt. %, preferably less than 25 wt. %, and particularly preferably less than 10 wt. % silicon nitride. In other words, in particular, ceramic elements are provided that are substantially free of silicon nitride.

[0026] 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 DGB process.

[0027] For example, it is provided that a method for producing a metal-ceramic substrate is provided, comprising:

[0028] - Providing a soldering layer, in particular in the form of at least one soldering foil or brazing foil,

[0029] - 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,

[0030] - 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 comprising 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

[0031] - 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.

[0032] In particular, a multi-layer soldering system comprising at least one solder layer, preferably free of melting point-lowering elements, particularly preferably a phosphorus-free solder layer, and at least one active metal layer is provided. The separation of the at least one active metal layer and the at least one solder layer proves to be particularly advantageous because it enables comparatively thin solder layers to be realized, especially when the solder layer is a foil. For solder materials containing active metals, comparatively large solder layer thicknesses must otherwise be realized due to the brittle intermetallic phases or the high modulus of elasticity and high yield strength of common active metals and their intermetallic phases, which hinder the forming of the solder paste or solder layer, whereby the minimum layer thickness is limited by the manufacturing properties of the solder material containing active metal.Accordingly, for solder layers containing active metals, the minimum solder layer thickness is not determined by the minimum thickness required for the joining process, but rather by the minimum technically feasible solder layer thickness. As a result, this thicker, active-metal-containing solder layer is more expensive than thinner layers. The term "phosphorus-free" is understood by those skilled in the art to mean, in particular, that the phosphorus content in the solder layer is less than 150 ppm, less than 100 ppm, and particularly preferably less than 50 ppm.

[0033] In particular, the use of a separately applied active metal layer makes it possible to make it comparatively thin, thereby achieving the claimed comparatively thin thicknesses of the bonding layer, in particular averaged over various thickness values ​​within the specified area(s). Examples of active metals 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. Furthermore, it should be noted that the elements Cr, Mo, and W are not classic active metals, but are suitable as a contact layer between SiSn4 and the at least one metal layer or the solder system or solder material, since they do not form intermetallic phases with the at least one metal layer, for example, copper, and do not have edge solubility.

[0034] Preferably, the solder layer, especially 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.

[0035] 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, particularly with a homogeneous distribution.

[0036] For example, in the production of the metal-ceramic substrate, in particular the metal-ceramic substrate, further steps are provided, comprising:

[0037] - Providing a ceramic element and a metal layer,

[0038] - providing a gas-tight container enclosing the ceramic element, wherein the container is preferably formed from the metal layer or comprises the metal layer,

[0039] - 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 at least partially between the metal layer and the ceramic element 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 another metal layer. Alternatively, it is also conceivable to use a glass container.

[0040] In hot isostatic pressing, it is particularly intended that bonding occurs by heating under pressure, during 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 present there, does not enter the melting phase. Accordingly, lower temperatures are required for hot isostatic pressing than for a direct metal bonding process, in particular a DGB process.

[0041] In comparison to the bonding of a metal layer to a ceramic layer by means of a solder material, which usually requires temperatures below the melting temperature of the at least one metal layer, 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 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 formation of shrinkage 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.

[0042] 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, ie lower than the melting temperature of the at least one metal layer.

[0043] It is preferably provided that during hot isostatic pressing, the metal container is exposed in a heating and pressure device to a gas pressure 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 allows the possibility of producing a process that is as void-free as possible, i.e.to produce a metal-ceramic substrate without gas inclusions between the metal layer and the ceramic element. In particular, process parameters mentioned in DE 2013 113 734 A1 are used, and are hereby explicitly referenced.

[0044] Furthermore, it is particularly preferred that the ceramic element, in particular the first grains and / or second grains, be monocrystalline. This has a particularly advantageous effect on the thermal conductivity of the metal-ceramic substrate.

[0045] 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. The ceramic element is provided by the following steps: providing granules comprising first grains with a first grain shape factor that 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 that is, on average, greater than 0.5, preferably greater than 0.6, and particularly preferably greater than 0.7; pressing, in particular isotropic pressing, the granules into a block shape; and sintering the block-shaped granules to form a ceramic block.

[0046] 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.

[0047] 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. Furthermore, it is conceivable that the sintering takes place at a temperature which assumes a value between 1600 °C and 2000 °C, preferably between 1700 °C and 1950 °C and particularly preferably between 1750 °C and 1900 °C. Furthermore, it is 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 which are isotropic, i.e. uniformly aligned.

[0048] 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 thickness of 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 include the surface properties on the top and back of the ceramic element, which correspond to one another when several ceramic elements are cut from a single ceramic block. This does not occur with conventional ceramic element manufacturing. Furthermore, it is conceivable to produce comparatively thin ceramic elements, particularly ceramic elements with a thickness of only 100 μm or less. Furthermore, it is possible to produce substrates with a thickness significantly greater than 1 mm. Furthermore, a diamond sawing technique, which is already established in the silicon wafer industry, can be used.

[0049] Preferably, at least one metal layer is bonded to the ceramic element and preferably structured to form conductor tracks. This advantageously makes it possible to use the produced ceramic element with the described properties as a metal-ceramic substrate used in power electronics, particularly as a printed 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.

[0050] Further advantages and features will become apparent from the following description of preferred embodiments of the subject matter of the invention with reference to the accompanying figures. They show:

[0051] Further advantages and features will become apparent from the following description of preferred embodiments of the subject matter of the invention with reference to the accompanying figures. They show:

[0052] Fig. 1 metal-ceramic substrate according to a first exemplary embodiment of the present invention;

[0053] Fig. 2 a detailed view of the grain

[0054] 13

[0055] REVISED SHEET (RULE 91) ISA / EP Fig. 3 a to 3 d schematic representation of a process for producing a metal-ceramic substrate

[0056] Figure 1 shows a metal-ceramic substrate 1 according to a first exemplary embodiment of the present invention. Such metal-ceramic substrates 1 preferably serve as carriers or circuit boards for electronic or electrical components, which can be bonded 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, i.e., in the manufactured metal-ceramic substrate 1, the at least one metal layer 10 comprises a plurality of metal sections that are electrically insulated from one another.The at least one metal layer 10, which extends 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, viewed 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'.

[0057] In this case, the at least one further metal layer 20 serves as a backside 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.

[0058] 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 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 advantageous if, for the purpose of structuring

[0059] REVISED SHEET (RULE 91) ISA / EP an etching process is provided for the at least one metal layer 10. For example, this allows for the realization of narrower isolation trenches, ie, distances between individual metal sections of the at least one metal layer 10.

[0060] 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.

[0061] In the example shown in Figure 1, 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 comprising 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, the thinnest possible design of this adhesion promoter layer 13 is advantageous 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 between the ceramic element 30 and the metal layer 10 for the bonding process and the bonding process is carried out 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 wt. %, preferably greater than 50 wt. %, and particularly preferably greater than 75 wt. %.

[0062] Figure 2 shows a first grain size consisting of a plurality of first grains 15 for the ceramic element 30 on its outer side, in particular on the outer side 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 ma x and a grain diameter Dörth perpendicular to Dmax. The grain diameter perpendicular to D ma x running grain diameter is measured halfway along D ma x, so that it is possible to determine a shape factor for each of the grains according to the relationship R= Dörth / D max. Preferably, the grains are measured on the surface or outer side or on a section plane of the ceramic element, which can be seen in a SEM image in a section through the metal-ceramic substrate 1.

[0063] 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, first grains 15 that have a substantially highly elliptical or non-circular shape. The grains 15 are elongated and have a main direction of extension. For example, the grains 15 are rice-grain-shaped.

[0064] In particular, it is provided that the first grains 15 in the ceramic element 30, whose average shape factor is less than 0.5, preferably less than 0.4, and particularly preferably less than 0.3, are isotropically aligned. 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 strengthening of the microstructure. At the same time, the highly elliptical first grains 15 support the development of a comparatively high thermal conductivity, preferably 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.

[0065] 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, wherein the second grains are assigned a shape factor or an average grain shape factor which, as an arithmetic mean, assumes a value greater than 0.6, preferably greater than 0.7, and particularly preferably greater than 0.8. The second grains are therefore those which 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, wherein an accumulation of the shape factors can be found in a first maximum at a value below 0.5 and a second maximum for the grain shape factors which have a value 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 the element into first grains 15 and second grains, rather than 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 thus 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 vary depending on the presence of the second grains and, in particular, the ratio of first grains to second grains in their number.

[0066] Preferably, the number of second grains and first grains is substantially equal. It is preferred that the 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 less than 0.2.

[0067] In the sectional view shown in Figure 2, only the first grains 15 are shown in order to determine a definition for the grain diameters.

[0068] Figures 3a-3d show a method for producing a ceramic element 30. In particular, the exemplary embodiment shown here provides that, in a first step, a granulate is produced by providing a powder, in particular an AlN powder, with first grains 15 or starting grains that have a shape 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 AlN whiskers, and are typically produced using a doctorblade process. Whiskers or AlN are preferably mixed together with sintering aids, such as Y2O3 or ZrO2, solvents (e.g., ethanol), and binders. The aim is not to crush these, but to comminute powder clusters. The resulting slurry is converted into granules in a spray tower.

[0069] Figure 3b shows the second process step. 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 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 on or 2 dm 3 and 8 dm 3 to.

[0070] Figure 3c shows the sintering step, the third process step, in which a ceramic block is created by applying pressure and temperature. The granulate now becomes a solid.

[0071] In order to utilize the ceramic material produced in this way to make ceramic elements 30 usable 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. Production using a corresponding block ceramic element and subsequent division with a sawing element, in particular with a diamond-enclosed wire saw, allows for the creation of ceramic elements with adjustable thickness. This allows for flexible selection of the thickness of the ceramic element 30 to be provided during production.

[0072] List of reference symbols:

[0073] 1 metal-ceramic substrate

[0074] 10 metal layer

[0075] 12 binding layer 12' further binding layer

[0076] 13 Adhesion promoter layer

[0077] 15 first grains

[0078] 20 additional metal layer 30 ceramic layer

Claims

Claims 1. Metal-ceramic substrate (1) as a carrier for electrical components, which serves in particular as a printed 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 ma x, one perpendicular to D ma x Dörth grain diameter, determined at half the length of D ma x, and a grain shape factor R= Dörth / 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) are aligned isotropically in the ceramic element (30).

2. Metal-ceramic substrate (1) according to claim 1, wherein the ceramic element (30) has a grain shape factor frequency distribution with at least two maxima, wherein a first maximum is assigned to the first grains (15) and a second maximum is assigned to second grains.

3. Metal-ceramic substrate (1) according to claim 1, wherein the first grains (15) and the second grains have the same ceramic material composition.

4. Metal-ceramic substrate (1) according to one of the preceding claims, 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 ohms / sq, preferably greater than 10 ohms / sq and particularly preferably greater than 20 ohms / sq.

5. 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.

6. Metal-ceramic substrate (1) according to one of the preceding claims, wherein the ceramic element (30) comprises aluminum nitride.

7. Metal-ceramic substrate (1) according to one of the preceding claims, wherein the ceramic element (30), in particular the first grains (15) and / or the second grains, comprises less than 50 wt.%, preferably less than 25 wt.% and particularly preferably less than 10 wt.% silicon nitride.

8. Metal-ceramic substrate (1), wherein a thermal conductivity of the ceramic element (30) is greater than 130 W / mK, preferably greater than 140 W / mK and particularly greater than 150 W / mK.

9. 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.

10. Metal-ceramic substrate (1) according to one of the preceding claims, wherein the first grains (15) are single-crystalline.

11. A 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, of the granulate into a block shape and - Sintering the block-shaped granules to form a ceramic block.

12. The method according to claim 11, 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.

13. 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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