DE102023103508A1 - Method for producing a metal-ceramic substrate and a metal-ceramic substrate produced by such a method - Google Patents

Abstract

Method for producing a metal-ceramic substrate (1) intended as a circuit board, comprising - providing a silicon wafer (40), preferably a doped or non-doped silicon wafer (40), - nitriding the silicon wafer (40) to produce a silicon nitride layer (31), - bonding a metal layer (10) to the silicon nitride layer (31) and - structuring the metal layer (10) to form a metallization.

Description

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

Metal-ceramic substrates are well known as printed circuit boards or circuit boards from the state of the art, for example from the EN 10 2013 104 739 A1 , the DE 19 927 046 B4 and the EN 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, wherein 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.

Particularly preferred ceramic elements for the formation of metal-ceramic substrates are silicon nitride ceramics, as they have a high insulation strength and a high thermal conductivity, which proves to be advantageous for the long-term use of a metal-ceramic substrate intended as a circuit board. For the use of silicon nitride ceramics as an insulation element in metal-ceramic substrates, reference is made, for example, to the US 6 242 374 B1 referred to.

Typically, silicon nitride ceramics are manufactured by producing a slurry-like flat product from silicon powder or granules, which is then nitrided to convert silicon into silicon nitride, i.e. in the form of a foil casting process. Subsequent sintering converts the flat product into a solid and stable body that can be used as a ceramic element in a metal-ceramic substrate. Examples of such a production process can be found in US 2016 0 362 592 A1 , CN 113 248 263 A and US10,669,210 B2 .

Based on the prior art, the present invention aims to provide metal-ceramic substrates that can be manufactured in a simplified manner.

The present invention solves this problem by a method for producing metal-ceramic substrates according to claim 1, a method according to claim 9 and a metal-ceramic substrate according to claim 10. Further embodiments can be found in the dependent claims, the description and the figures.

• - Bereitstellen einer Siliziumscheibe, vorzugsweise einer dotierten oder einer nicht-dotierten Siliziumscheibe,
• - Nitridieren der Siliziumscheibe zum Erzeugen einer Siliziumnitridlage,
• - Anbinden einer Metallschicht an die Silizumnitridlage und
• - Strukturierung der Metallschicht zur Ausbildung einer Metallisierung.

According to a first aspect, a method for producing a metal-ceramic substrate intended as a circuit board is proposed, comprising

• - Providing a silicon wafer, preferably a doped or non-doped silicon wafer,
• - Nitriding the silicon wafer to produce a silicon nitride layer,
• - Bonding a metal layer to the silicon nitride layer and
• - Structuring the metal layer to form a metallization.

In contrast to the methods known from the prior art, a silicon nitride layer is proposed here which is created by nitriding a silicon wafer, in particular in the outer region of the silicon wafer. This advantageously makes it possible to use industrially manufactured silicon wafers which are available in large quantities as standard. Nitriding converts the silicon, at least partially, into silicon nitride, which ensures the desired insulation strength for the nitrided silicon wafer. In particular, it is provided that the nitriding is implemented in such a way that the resulting silicon nitride layer has an insulation strength which is greater than 20 kV/mm, preferably greater than 50 kV/mm and particularly preferably greater than 100 kV/mm. This advantageously makes it possible to provide a ceramic element containing silicon nitride in a cost-effective and simple manner, which is used for insulation in a metal-ceramic substrate. The nitrided silicon wafer serves as a ceramic element. In particular, the present invention understands the nitrided silicon wafer as a ceramic element even if the nitrided silicon wafer is not completely nitrided, for example if it has predominantly silicon in the core. Preferably, the, in particular completely nitrided, silicon wafer or the silicon nitride layer comprises a proportion of silicon nitride that is greater than 60% by weight, preferably greater than 75% by weight and particularly preferably greater than 90% by weight. It is also conceivable that a partially nitrided silicon wafer has a proportion of silicon nitride that is greater than 2% by weight, preferably greater than 5% by weight and particularly preferably greater than 10% by weight, in particular related to the entire ceramic element, which has silicon in its core.

For nitriding, it is preferably provided that the silicon wafer is placed in a furnace, in particular in a gas sintering furnace, and exposed to a nitrogen atmosphere. It is particularly preferred if a pressure of at least 0.2 MPa, preferably more than 0.5 MPa and particularly preferably more than 0.8 MPa prevails or is present in the furnace. It is also preferred if the silicon wafer is exposed to the nitrogen atmosphere for at least between 0.5 and 10 hours, preferably 1 and 5 hours and particularly preferably between 1.5 and 3 hours. Temperatures can preferably be between 1000 °C and 2000 °C, preferably between 1200 °C and 1800 °C and particularly preferably between 1300 °C and 1500 °C. It has also been found that the nitriding temperature does not affect the conductivity.

Preferably, the silicon wafer is heated in several stages, at least two stages, to prevent the high temperatures from melting the silicon wafer. This preferably results in α-silicon nitride being formed first, which is then converted into β-silicon nitride in the second temperature stage.

In particular, it is intended that the final conversion of the silicon preferably takes place exclusively in β-silicon nitride. For example, a ratio of α-silicon nitride to β-silicon nitride is less than 0.2, preferably less than 0.1 and particularly preferably less than 0.05

Furthermore, it is particularly preferred if a grain size associated with the silicon grains is smaller than 250 µm, preferably smaller than 100 µm and particularly preferably smaller than 50 µm. It has been found that the conversion efficiency during nitriding increases with decreasing grain size. This allows the efficiency of converting silicon into silicon nitride to be increased accordingly.

The silicon wafer is preferably a single-crystal or monocrystalline or a multicrystalline body, which is particularly preferably flat and extends along a main extension plane. In particular, it is a silicon wafer. In particular, the nitrided silicon wafer is also monocrystalline. Preferably, the silicon wafer is not a green body, i.e. the silicon wafer does not have a sludge-like consistency, but is to be understood as a self-supporting body that does not deform when simply lifted on one side, but remains essentially dimensionally stable.

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 bending 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, CuAI 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. 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 an edge casting on the metallization to suppress crack formation or widening is conceivable as a surface modification.

It is preferably provided that a further silicon nitride layer is formed on the silicon wafer during nitriding on a side opposite the at least one silicon wafer. This advantageously makes it possible to form silicon nitride layers on opposite sides, which enables the connection of a metal layer and a further metal layer on the opposite sides. This allows a metal layer and a further metal layer, which serves as rear-side metallization in the finished metal-ceramic substrate, to be bonded to the ceramic element. The rear-side metallization serves in particular the purpose of symmetrizing the top and bottom of the metal-ceramic substrate in order to to counteract bending. By forming a silicon nitride layer together, the same active solder material can be used, which means that metal layer and further metal layer can be bonded to the nitrided silicon wafer, which serves as a ceramic element for the finished substrate, in a single work step.

Preferably, it is provided that an adhesive strength with which the metal layer is bonded to the ceramic element, in particular to the silicon nitride layer, is comparable in size to an adhesive strength with which the further metal layer is bonded to the ceramic element, in particular to the further silicon nitride layer. For example, the adhesive strengths do not differ from each other in their measurable size by more than 5%, preferably not more than 2.5% and particularly preferably not more than 1%. This achieves as symmetrical an adhesive strength as possible for the component side and the back of the metal-ceramic substrate. This is caused by the top and bottom of the nitrided silicon nitride disk having the same surface specifications if they are both exposed to the same amount of nitrogen during nitriding. For this purpose, the silicon disk is preferably arranged in the furnace in such a way, for example with a corresponding holder, that two opposite sides of the silicon disk are directly exposed to the nitrogen. This distinguishes them from state-of-the-art silicon nitride ceramics, which are manufactured as a slip-like mass on a processing surface and are more difficult to access for nitrogen on one side. In such state-of-the-art ceramic elements, the surface specifications on the opposite sides therefore differ from those of the silicon wafer nitrided on both sides.

Furthermore, the manufactured metal-ceramic substrate differs from those known from the prior art in that the ceramic element is essentially free of an accumulation of sintering agents in the outer circumference of the ceramic element. In the case of ceramic elements formed by foil casting, a layer approximately 20 µm to 30 µm thick is finally formed in which sintering agents accumulate and diffuse there during the sintering process. This is referred to as the so-called sinter skin. Examples of sintering agents are MgO or Y2O3. In contrast, the sintering agents in the silicon nitride layer in the nitrided silicon wafer are essentially homogeneously distributed.

It is preferably provided that the silicon wafer has a first thickness, wherein the at least one silicon nitride layer has a second thickness and the further silicon nitride layer has a third thickness, wherein a ratio of the second thickness and/or third thickness to the first thickness is less than 0.5, preferably less than 0.2 and particularly preferably less than 0.1. This expresses that the silicon wafer does not have to be completely nitrided, but it is already sufficient if nitriding takes place on the outermost circumference of the silicon wafer, preferably all the way around, and a silicon nitride layer and/or a further silicon nitride layer is formed starting on the outside of the silicon wafer. Preferably, a continuous silicon nitride layer is formed all the way around the entire outside of the silicon wafer, which extends both on the top and on the bottom, i.e. on opposite sides of the silicon nitride wafer. The second and/or third thickness is preferably set in such a way that an insulation thickness of the silicon nitride layer that is sufficient for the application is provided.

Preferably, the silicon nitride layer is completely nitrided. Partial nitriding of the silicon wafer is particularly advantageous when sufficient insulation strength is to be achieved as quickly as possible. This shortens the work process, particularly nitriding, and also the effort involved in producing a nitrided silicon wafer that can be used as a ceramic element. Complete nitriding is particularly advantageous when comparatively thin silicon wafers are provided, since in this case complete nitriding can already be carried out and insulation strength can thus be ensured, or when high insulation strengths are required.

Furthermore, it is preferably provided that the first thickness is less than 1 mm, preferably less than 0.5 mm and particularly preferably less than 0.25 mm.

This provides silicon wafers that can be used directly as a ceramic element in a metal-ceramic substrate.

Furthermore, it is provided that the silicon layer has a roughness that is greater than 0.2 µm, preferably greater than 1.0 µm and particularly preferably greater than 2.0 µm. This advantageously makes it possible to increase the surface area of the silicon wafer, whereby the conversion efficiency during nitriding can be increased.

It is particularly preferred that the silicon layer has a roughness that is less than 1.0 µm, preferably less than 0.5 µm and particularly preferably less than 0.2 µm. This creates a particularly smooth, particularly saw-smooth, surface that is nitrided. This proves to be particularly advantageous when no further work step is required to process the silicon wafer.

Furthermore, it is intended that the surface side of the silicon wafer to be nitrided has a surface profile. For example, it is conceivable that the silicon wafer has grooves or slots that have a triangular or square cross-sectional shape, for example. As a result, the surface can be specifically adapted to the optimal conditions for nitriding. In addition, the recesses can be used to collect active solder material when the nitrided silicon wafer is used to attach a metal layer to it.

In order to produce the desired roughness and/or profiling in the silicon wafer, it is preferably provided that a preparation step is provided in which, before nitriding, the surface or outside of the silicon wafer is roughened and/or profiled at least in sections in order to enlarge the surface. This provides a larger area that can be used for later bonding the metal layer to the ceramic element.

It is preferably provided that the metal layer is bonded to the silicon nitride layer by means of an active soldering process and/or hot isostatic pressing to form a metal-ceramic substrate. This ensures a permanent bond between the metal layer and the silicon nitride layer, which is particularly advantageous for circuit boards. It is also preferably provided that the metal-ceramic substrate has a metallization, i.e. a structured metal layer in which individual metal sections are electrically insulated from one another.

• - Bereitstellen einer Lötschicht, insbesondere in Form mindestens einer Lötfolie bzw. Hartlotfolie,
• - Beschichten des Keramikelements, insbesondere der nitrierten Siliziumscheibe, 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, in particular the nitrided silicon wafer, 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 forming 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, the minimum thickness of the solder layer is not determined by the minimum thickness required for the joining process, but rather by the minimum thickness of the solder layer that is technically feasible. This makes this thicker solder layer containing active metals 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 claimed comparatively thin thicknesses of the bonding layer can be achieved, in particular averaged over different measurement values within the specified area or areas. For example, the active metal layer is thinner than 25 µm, preferably thinner than 18 µm and particularly preferably thinner than 12 µm or even thinner than 1000 nm, for example between 400 nm and 800 nm. 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 can be used 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 do not form intermetallic phases with the at least one metal layer, for example copper, and do not have any edge solubility.

Preferably, a proportion of active metal in an adhesion promoter layer comprising active metal or the active metal layer is greater than 15% by weight, preferably greater than 20% by weight and particularly preferably greater than 25% by weight.

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. The solder layer or a solder base material preferably has a proportion of active metal that is less than 1.5% by weight, preferably less than 1.0% by weight and particularly preferably less than 0.5% by weight. In particular, the solder base material is free of active metal.

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 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 been found advantageously that it is thus possible to attach a metal layer, ie a first and/or second metal layer of the metal container, to the ceramic element. bond 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 of a corresponding gas pressure makes it possible to produce a metal-ceramic substrate with as few cavities as possible, i.e. without gas inclusions between the metal layer and the ceramic element. In particular, process parameters are used that are used in the DE 2013 113 734 A1 mentioned and to which explicit reference is hereby made.

• - Bereitstellen einer Siliziumscheibe, vorzugsweise einer dotierten oder einer nicht-dotierten Siliziumscheibe,
• - Nitridieren der Siliziumscheibe zum Erzeugen einer Siliziumnitridlage. Alle zum Verfahren zur Herstellung des Metall-Keramik-Substrats beschriebenen Eigenschaften und Vorteile lassen sich analog übertragen auf das Verfahren zur Herstellung eines elektrisch isolierenden Keramikelements und andersrum.

According to a further aspect, there is provided a method for producing an electrically insulating ceramic element, comprising:

• - Providing a silicon wafer, preferably a doped or non-doped silicon wafer,
• - Nitriding the silicon wafer to produce a silicon nitride layer. All properties and advantages described for the process for producing the metal-ceramic substrate can be transferred analogously to the process for producing an electrically insulating ceramic element and vice versa.

A further aspect of the present invention is a metal-ceramic substrate which is produced using the method according to the invention. All properties and advantages which are described together with the method apply analogously to the subject matter of the method and vice versa.

In particular, the correspondingly manufactured metal-ceramic substrate is characterized by the fact that the adhesive strength of the metal layer to the ceramic element and the adhesive strength of the further metal layer to the ceramic element are essentially the same. In addition, no residues can be seen on the ceramic element that are attributable to a separating layer, in particular to a separating layer comprising boron nitride. Such separating agents are used in the processes known from the prior art, in particular in film casting processes, to stack several green compacts on top of each other. They cannot be removed from the ceramic element without leaving residues and remain on the ceramic element as a sintered skin. Finally, they can still be detected on the manufactured metal-ceramic substrate. This does not apply to the metal-ceramic substrate manufactured according to the invention.

According to a preferred embodiment of the present invention, it is provided that in the manufactured metal-ceramic substrate a bonding layer is formed between the metal layer and the ceramic element, in particular the silicon nitride layer, wherein an adhesion promoter 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.

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 as the proportion of active metal in the bonding layer decreases. 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 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. 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. Using a four-point measurement, a surface 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 surface resistance of a material sample is to be understood as its resistance in relation to a square surface area. It is usual to describe the surface resistance with the unit Ohm/sq(square). The physical unit of the surface resistance is Ohm. Preferably it is 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 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 multiple areas are mentioned, this means in particular that the at least one metal layer is divided into areas that are as equal in size as possible and that in each of these areas dividing the at least one metal layer, at least one value, preferably several measured values, for the thickness are recorded. The thicknesses determined in this way 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 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, 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, the bonding layer is formed over a flat area, in particular without interruption, i.e. continuously, between the at least one metal layer and the ceramic element. Preferably, it is provided that a ratio of an area in which no bonding layer is formed between the at least one metal layer and the ceramic element to the areas in which a bonding layer is formed between the at least one bonding layer and the ceramic element is less than 0.05 mm, preferably less than 0.02 mm and particularly preferably less than 0.007 mm. The person skilled in the art will understand in particular that in order to form this ratio, the areas that are free of metal of the at least one metal layer due to the structuring are not taken into account.

It is preferably provided that the thermal conductivity of the ceramic element is greater than 50 W/mK, preferably greater than 80 W/mK and particularly preferably greater than 100 W/mK. It is preferably provided that the ceramic element is present as a composite of a silicon disk, in particular as a wafer, which is nitrided on the outside. Correspondingly high thermal conductivities also prove to be particularly advantageous for the dissipation of heat, in particular in addition to the roughness of the surface mentioned above. This prevents the heat from being stored in the ceramic element after the heat has been quickly dissipated via the interface between the ceramic element and the metal layer. This means that corresponding metal-ceramic substrates can be used particularly advantageously for high-performance electronic components that develop a very high level of heat during operation.

• 1: schematusche Darstellung eines Metall-Keramik-Substrats;
• 2 schematische Darstellung eines Verfahrens zur Herstellung eines Metall-Keramik-Substrats gemäß einer ersten Ausführungsform der vorliegenden Erfindung und
• 3 schematische Darstellung eines Verfahrens zur Herstellung eines Metall-Keramik-Substrats gemäß einer zweiten Ausführungsform der vorliegenden Erfindung

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 : schematic representation of a metal-ceramic substrate;
• 2 schematic representation of a method for producing a metal-ceramic substrate according to a first embodiment of the present invention and
• 3 schematic representation of a method for producing a metal-ceramic substrate according to a second embodiment of the present invention

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 to form a metallization. The at least one metal layer extending essentially along a main extension plane HSE 10 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. In addition to the at least one metal layer 10, the metal-ceramic substrate 1 preferably comprises 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 connected 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 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 between the ceramic element 30 and the metal layer 10 for the bonding process and the bonding process takes place via hot isostatic pressing. The adhesion promoter layer 13 can, however, 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 2 is a schematic representation of the production of a metal-ceramic substrate 1 serving as a circuit board according to a first exemplary embodiment. In particular, it is provided that a silicon disk 40 is first provided to provide an insulating ceramic element 30. In particular, this is a silicon disk 40, i.e. a body made of silicon that runs essentially flat along a main extension plane HSE, particularly preferably a silicon wafer. The silicon disk 40 can be doped or undoped. The silicon disk 40 preferably has a silicon content of more than 50% by weight, preferably more than 70% by weight and particularly preferably more than 80% by weight.

Furthermore, it is preferably provided that a first thickness D1 of the silicon wafer 40, measured in a direction perpendicular to the main extension plane HSE, is less than 4 mm, preferably less than 2 mm and particularly preferably less than 1 mm. In particular, the silicon wafer 40 is a silicon wafer that is manufactured as standard and is preferably round. It is conceivable that an outer side, in particular a surface of the silicon wafer 40, has a roughness that is greater than X1 µm, preferably greater than X2 µm and particularly preferably greater than X3 µm.

This advantageously increases the surface area of the silicon wafer 40. Alternatively or additionally, it is conceivable that a profiling is and/or will be formed on the outside or surface of the silicon wafer 40 in order to increase the surface area of the silicon wafer 40. It is also conceivable that the silicon wafer 40 is roughened. Alternatively, it is conceivable that the roughness on the surface is less than Y1 µm, preferably less than Y2 µm and particularly preferably less than Y3 µm. It is also conceivable that the silicon wafer 40 has sections with a first roughness and regions with a second roughness, wherein the first roughness is smaller than the second roughness. In particular, a surface roughness is modeled along the outside of the silicon wafer 40. For example, it is also conceivable that the first roughness is formed on a first outside of the silicon wafer 40 and a second roughness on a second side of the silicon wafer 40 opposite the first side.

Furthermore, it is conceivable that the silicon wafer 40 is a monocrystalline body or a multicrystalline body. Furthermore, it is conceivable that a grain size of the silicon particles in the silicon wafer 40 is smaller than 250 µm, preferably smaller than 100 µm and particularly preferably smaller than 50 µm.

The method, in particular for providing the ceramic element 30, which is subsequently used for forming a metal-ceramic substrate 1, provides for at least partial nitriding of the silicon wafer 40 in a second method step. For this purpose, the silicon wafer 40 is introduced into a furnace 5, in particular into a gas inter-furnace, in order to be nitrided there. The silicon wafer 30 is exposed to a temperature between 1000 °C and 2000 °C, preferably between 1100 °C and 1800 °C and particularly preferably between 1300 °C and 1500 °C. In this case, a pressure of at least 0.2 MPa, preferably of at least 0.7 MPa and particularly preferably of at least 0.9 MPa prevails in the gas inter-furnace. In particular, a nitrogen atmosphere prevails in the gas inter-furnace, with a nitrogen proportion being at least greater than 85%, preferably greater than 90% and particularly preferably greater than 95%. The conversion efficiency during nitriding is positively influenced by the smallest possible grain size in the silicon wafer. Nitriding is carried out in particular by silicon nitride being formed by the diffusion of nitrogen into a solid silicon body.

In the 2 In the embodiment shown, the entire silicon wafer 40 is not completely nitrided, but at least one silicon nitride layer 31 and a further silicon nitride layer 32 are formed on the top and bottom of the silicon wafer 40, i.e. on the opposite sides of the silicon wafer 40. The at least one silicon nitride layer 31 has a second thickness D2 and the further silicon nitride layer 32 has a third thickness D3, the second thickness D2 and/or the third thickness D3 being smaller than the first thickness D1 of the silicon wafer 30, which expresses that complete nitriding, i.e. of the entire silicon wafer 30, has not taken place. In particular, the nitriding is carried out in such a way that a silicon nitride layer 31 is formed whose insulation strength is sufficient to use the nitrided silicon wafer as a ceramic element 30 in the metal-ceramic substrate 1. It is conceivable that the second thickness D2 corresponds to the third thickness D3. Alternatively, it is conceivable that the second thickness D2 differs from the third thickness D3, in particular by setting different roughnesses and/or profiles on the first side and the second side of the silicon wafer 30 opposite the first side, whereby the conversion efficiency during nitriding can be influenced. This also leads to different adhesive strengths of the metal layer and the further metal layer on the opposite sides of the ceramic element. It is also conceivable that the silicon wafer 40 lies on a covering surface in the furnace, so that only reduced nitriding takes place on at least one side. In such a case, essentially only at least one silicon nitride layer 31 is formed, which in the ceramic-metal substrate 2 faces the at least one metal wafer 10, which is structured to form the conductor track and/or connection surfaces.

After nitriding, at least one metal layer 10 is bonded to the at least one silicon nitride layer 31, preferably using an active soldering process and/or hot isostatic pressing. This advantageously makes it possible to enable the metal layer 10 to be applied to the ceramic element 30, in particular to the silicon nitride layer 31. Preferably, in addition to the silicon nitride layer 31, a further silicon nitride layer 32 is formed on the opposite side of the silicon wafer 40 and, in particular, a metal layer 10 and a further metal layer 20 opposite the metal layer 10 are bonded to the nitrided silicon wafer in a common bonding process, for example using the same active soldering process and material. This ensures the desired symmetry in order to counteract bending.

After the at least one metal layer 10 or the at least one further metal layer 20 is bonded to the nitrided silicon wafer serving as ceramic element 30, the at least one metal layer is preferably structured in order to provide connection surfaces and/or conductor tracks which are intended for use of the metal-ceramic substrate 1 as a circuit board.

In 3 A method according to a second exemplary embodiment of the present invention is shown schematically. The embodiment of the 3 essentially only in this respect from the embodiment of the 2 that complete nitriding of the silicon wafer 40 takes place. In other words: the silicon wafer 40 is nitrided until a continuous silicon nitride layer 31 is formed. It is preferably provided that a proportion of silicon nitride in the nitrided silicon wafer or silicon nitride layer is greater than 80%, preferably greater than 90% and particularly preferably greater than 95%. It is also conceivable that a partially nitrided silicon wafer has a proportion of silicon nitride that is greater than 2% by weight, preferably greater than 5% by weight and particularly preferably greater than 10% by weight, based on the entire ceramic element that has silicon in the core.

Reference number:

1

Metal-ceramic substrate

5

Oven

10

Metal layer

12

Binding layer

12`

additional binding layer

13

Adhesion promoter layer

20

additional metal layer

30

Ceramic element

31

Silicon nitride layer

32

additional silicon nitride layer

40

Silicon wafer

HSE

Main extension level

S

Stacking direction

D1

first thickness

D2

second thickness

D3

third thickness

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 102013104739 A1 [0002]
• DE 19927046 B4 [0002]
• DE 102009033029 A1 [0002]
• US 6242374 B1 [0003]
• US 20160362592 A1 [0004]
• CN113248263A [0004]
• US 10669210 B2 [0004]
• DE 2013113734 A1 [0038]

Claims (10)

Method for producing a metal-ceramic substrate (1) intended as a circuit board, comprising - providing a silicon wafer (40), preferably a doped or non-doped silicon wafer (40), - nitriding the silicon wafer (40) to produce a silicon nitride layer (31), - bonding a metal layer (10) to the silicon nitride layer (31) and - structuring the metal layer (10) to form a metallization. Method according to one of the preceding claims, wherein a further silicon nitride layer (32) is formed on the silicon wafer (40) during nitriding on a side opposite the silicon nitride layer (31). Method according to one of the preceding claims, wherein the silicon wafer (40) has a first thickness (D1), wherein the silicon nitride layer (31) has a second thickness (D2) and preferably the further silicon nitride layer (32) has a third thickness (D3), wherein a ratio of the second thickness (D2) and/or the third thickness (D3) to the first thickness (D1) is less than 0.5, preferably less than 0.2 and particularly preferably less than 0.1. Method according to one of the preceding claims, wherein the silicon wafer (40) is completely nitrided. Method according to one of the preceding claims, wherein the first thickness (D1) is less than 4 mm, preferably less than 2 mm and particularly preferably less than 1 mm. Method according to one of the preceding claims, wherein the silicon layer (40) has a roughness that is greater than 0.2 µm, preferably greater than 1.0 µm and particularly preferably greater than 2.0 µm. Method according to one of the preceding claims, wherein the silicon layer (40) has a roughness which is less than 1.0 µm, preferably less than 0.5 µm and particularly preferably less than 0.2 µm. Method according to one of the preceding claims, wherein a surface side of the silicon wafer (40) to be nitrided has a surface profiling. Method for producing an electrically insulating ceramic element, comprising: - providing a silicon wafer (40), preferably a doped or non-doped silicon wafer (40), - nitriding the silicon wafer (40) to produce a silicon nitride layer (31). Metal-ceramic substrate (1) produced by a process according to one of the Claims 1 until 8 .

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