Process for producing a metal-ceramic substrate, and a metal-ceramic substrate produced using such a process

Abstract

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

Description

[0001] Method for producing a metal-ceramic substrate and a metal-ceramic substrate produced by such a method

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

[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 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 interconnected to form electrical circuits. Essential components of the metal-ceramic substrates are an insulating layer, which is 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] Particularly preferred ceramic elements for the formation of metal-ceramic substrates are silicon nitride ceramics, as they have high insulation strength and high thermal conductivity, which proves advantageous for the long-term use of a metal-ceramic substrate intended as a circuit board. Regarding the use of silicon nitride ceramics as insulation elements in metal-ceramic substrates, reference is made, for example, to US Pat. No. 6,242,374 B1. 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 a tape-casting process. Subsequent sintering transforms the flat product into a solid and stable body that can be used as a ceramic element in a metal-ceramic substrate.Corresponding examples of such production can be found in US 2016 0 362 592 A1 , CN 113 248 263 A and US 10,669,210 B2.

[0005] Based on the prior art, the present invention aims to provide metal-ceramic substrates that can be manufactured in a simplified manner.

[0006] 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.

[0007] According to a first aspect, a method for producing a metal-ceramic substrate intended as a printed circuit board is proposed, comprising

[0008] - Providing a silicon wafer, preferably a doped or non-doped silicon wafer,

[0009] - Nitriding the silicon wafer to produce a silicon nitride layer,

[0010] - Bonding a metal layer to the silicon nitride layer and

[0011] - Structuring the metal layer to form a metallization.

[0012] 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. The nitriding converts the silicon, at least partially, into silicon nitride, thereby ensuring 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 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, which is used for insulation in a metal-ceramic substrate, in a cost-effective and simple manner. The nitrided silicon wafer serves as the ceramic element. In particular, the present invention also understands the nitrided silicon wafer as a ceramic element if the nitrided silicon wafer is not completely nitrided, for example has predominantly silicon in the core. The, in particular completely nitrided, silicon wafer or the silicon nitride layer preferably comprises a proportion of silicon nitride that is greater than 60 wt. %, preferably greater than 75 wt. % and particularly preferably greater than 90 wt. %. It is also conceivable for a partially nitrided silicon wafer to have a proportion of silicon nitride that is greater than 2 wt. %, preferably greater than 5 wt. % and particularly preferably greater than 10 wt.- %, in particular based on the entire ceramic element which has silicon in the core.

[0013] For nitriding, it is preferably provided that the silicon wafer is placed in a furnace, in particular 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. Furthermore, it is preferred if the silicon wafer is exposed to the nitrogen atmosphere for at least between 0.5 and 10 hours, preferably 1 to 5 hours and particularly preferably between 1.5 and 3 hours. Temperatures between 1000 °C and 2000 °C, preferably between 1200 °C and 1800 °C and particularly preferably between 1300 °C and 1500 °C can prevail. It has also been found that the nitriding temperature does not impair conductivity.Nitriding converts at least a portion of the silicon into silicon nitride, forming a layer that can be used as an insulating ceramic element in the metal-ceramic substrate. The entire, possibly only partially nitrided, silicon wafer is then considered a ceramic element for the metal-ceramic substrate, even if the nitrided silicon wafer contains sub-regions that are not technically considered purely ceramic.

[0014] 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 initially forms α-silicon nitride, which is then converted into β-silicon nitride in the second temperature stage.

[0015] In particular, it is intended that the final conversion of the silicon preferably takes place exclusively into ß-silicon nitride. For example, a ratio of α-silicon nitride to ß-silicon nitride is less than 0.2, preferably less than 0.1, and particularly preferably less than 0.05.

[0016] Furthermore, it is particularly preferred if the grain size associated with the silicon grains is less than 250 pm, preferably less than 100 pm, and particularly preferably less than 50 pm. 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.

[0017] The silicon wafer is preferably a single-crystalline or monocrystalline or a multi-crystalline body, which is particularly preferably flat and extends along a main plane of extension. In particular, it is a silicon wafer. In particular, the nitrided silicon wafer is also monocrystalline. Preferably, the silicon wafer is not a green compact, i.e. the silicon wafer does not have a sludge-like consistency, but is to be understood as a self-supporting body which does not deform when simply lifted on one side, but rather remains essentially dimensionally stable. In particular, a monocrystalline silicon wafer differs in its chemical and physical properties from a silicon wafer which is produced by pressing and / or sintering silicon powder in order to form the silicon wafer.Preferably, at least a portion of the silicon is converted into silicon nitride to provide the insulating ceramic element required for the metal-ceramic element. In particular, the silicon wafer is not a silicon wafer produced by sintering powder and / or by a foil casting process.

[0018] 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.

[0019] Conceivable materials for the at least one metal layer and / or the at least one further metal layer in the metal-ceramic substrate 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. Possible surface modifications include, for example, sealing with a precious metal, in particular silver and / or gold, or (electroless) nickel or ENIG (“electroless nickel immersion gold”), or edge encapsulation on the metallization to suppress crack formation or crack widening.

[0020] It is preferably provided that, during nitriding, a further silicon nitride layer is formed on the silicon wafer 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 bonding 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 backside metallization in the finished metal-ceramic substrate, to be bonded to the ceramic element. The backside metallization serves, in particular, the purpose of symmetrizing the top and bottom sides of the metal-ceramic substrate in order to counteract bending.By forming a silicon nitride layer together, the same active solder material can be used, allowing the metal layer and another metal layer to be bonded to the nitrided silicon wafer, which serves as a ceramic element for the finished substrate, in a single step.

[0021] Preferably, the bond strength with which the metal layer is bonded to the ceramic element, in particular to the silicon nitride layer, is comparable in magnitude to the bond strength with which the further metal layer is bonded to the ceramic element, in particular to the further silicon nitride layer. For example, the bond strengths differ from one another in their measurable magnitude by no more than 5%, preferably no more than 2.5%, and particularly preferably no more than 1%. This achieves the most symmetrical bond strength possible for the component side and the back of the metal-ceramic substrate. This is achieved by ensuring that the top and bottom sides of the nitrided silicon nitride wafer have the same surface specifications if they are both exposed to nitrogen to the same extent during nitriding.For this purpose, the silicon wafer is preferably arranged in the furnace, for example using a suitable holder, such that two opposite sides of the silicon wafer, intended for connection to the metal layer and / or further metal layer, are directly exposed to the nitrogen. The opposite sides extend in particular along a plane parallel to the main extension plane. This distinguishes it from state-of-the-art silicon nitride ceramics, which, during production, rest on a processing surface as a slurry-like mass and are less accessible to nitrogen on one side. Therefore, in such state-of-the-art ceramic elements, the surface specifications on the opposite sides differ from those of the double-sided nitrided silicon wafer.

[0022] Furthermore, the manufactured metal-ceramic substrate differs from those known from the prior art in that the ceramic element, i.e., the nitrided silicon wafer, is essentially free of an accumulation of sintering agents in the outer periphery of the ceramic element, the nitrided silicon wafer. In the case of ceramic elements formed by tape casting, a layer approximately 20 μm to 30 μm thick ultimately forms, in which sintering agents accumulate and diffuse there during sintering. This is referred to as the sinter skin. Examples of sintering agents include MgO or Y2O3. In contrast, the sintering agents are essentially homogeneously distributed in the silicon nitride layer in the nitrided silicon wafer.

[0023] 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 complete nitriding of the silicon wafer does not have to take place, but it is already sufficient if nitriding takes place on the outermost circumference of the silicon wafer, preferably circumferentially, and a silicon nitride layer and / or a further silicon nitride layer is formed starting on the outer side of the silicon wafer. Preferably, a continuous silicon nitride layer is formed circumferentially over the entire outer side of the silicon wafer, which is located both on the upper side and on the underside, ieon opposite sides of the silicon nitride wafer. The second and / or third thickness is preferably adjusted in such a way that the silicon nitride layer provides sufficient insulation thickness for the application.

[0024] In particular, a soft core is created between the silicon nitride layer and the further silicon nitride layer in the intermediate region between the component metallization and the rear-side metallization. It is also conceivable for the core region between the silicon nitride layer and the further silicon nitride layer to be given semiconductor capability through doping, for example with boron, phosphorus, and / or nitrogen. It has also been advantageously found that by adjusting the ratio of the first thickness, the second thickness, and the third thickness to one another, the thermal shock resistance of the produced metal-ceramic substrate can be specifically influenced. In addition, the soft, non-nitrided core of the silicon wafer can be specifically shielded. Furthermore, it is preferably provided that the partially nitrided silicon wafer has a conductivity of at least 60 W / mK.

[0025] Preferably, the silicon nitride layer is fully 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, especially the nitriding process, and also the effort required to produce a nitrided silicon wafer that can be used as a ceramic element. Full nitriding is particularly advantageous when comparatively thin silicon wafers are provided, since in this case, complete nitriding can already be achieved, thus ensuring insulation strength, or when high insulation strengths are required.

[0026] 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 pm, preferably greater than 1.0 pm, and particularly preferably greater than 2.0 pm. This advantageously makes it possible to increase the surface area of ​​the silicon wafer, whereby the conversion efficiency during nitriding can be increased.

[0027] Particularly preferably, the silicon layer has a roughness of less than 1.0 pm, preferably less than 0.5 pm, and particularly preferably less than 0.2 pm. This provides a particularly smooth, in particular saw-smooth, surface that is nitrided. This proves particularly advantageous when no further processing step is required to process the silicon wafer.

[0028] Furthermore, the surface of the silicon wafer to be nitrided is provided with a surface profile. For example, it is conceivable for the silicon wafer to have grooves or notches with, for example, a triangular or square cross-sectional shape. Consequently, the surface can be specifically adapted to the optimal conditions for nitriding. Furthermore, the recesses can be used to collect active solder material when the nitrided silicon wafer is used to bond a metal layer to it.

[0029] To create the desired roughness and / or profiling in the silicon wafer, it is preferably provided that a preparation step is provided in which, prior to nitriding, the surface or outer side of the silicon wafer is roughened and / or profiled at least in sections to enlarge the surface area. This provides a larger area that can be used for later bonding the metal layer to the ceramic element.

[0030] Preferably, the metal layer is bonded to the silicon nitride layer using 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 printed circuit boards. Furthermore, it is 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.

[0031] For example, it is provided that a method for producing a metal-ceramic substrate is provided, comprising:

[0032] - Providing a soldering layer, in particular in the form of at least one soldering foil or brazing foil,

[0033] - 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,

[0034] - 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

[0035] - 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.

[0036] 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.

[0037] In particular, the use of a separately applied active metal layer makes it possible to make it comparatively thin, thereby realizing the claimed comparatively thin thicknesses of the bonding layer, in particular averaged over various thickness values ​​within the specified area or areas. For example, the active metal layer is thinner than 25 pm, preferably thinner than 18 pm, and particularly preferably thinner than 12 pm 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. Furthermore, it is noted that the elements Cr, Mo and W are not classic active metals, but can act as a contact layer between SisN4 and the at least one metal layer or the solder system.Solder materials are suitable because they do not form intermetallic phases with at least one metal layer, for example copper, and do not have edge solubility.

[0038] Preferably, a proportion of active metal in an adhesion promoter layer comprising active metal or the active metal layer is greater than 15 wt.%, preferably greater than 20 wt.% and particularly preferably greater than 25 wt.%.

[0039] The solder layer, in particular the phosphorus-free solder layer, preferably 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 an active metal content of less than 1.5 wt. %, preferably less than 1.0 wt. %, and particularly preferably less than 0.5 wt. %. In particular, the solder base material is free of active metal.

[0040] 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.

[0041] For example, in the production of the metal-ceramic substrate, in particular the metal-ceramic substrate, further steps are provided, comprising:

[0042] - Providing a ceramic element and a metal layer,

[0043] - providing a gas-tight container enclosing the ceramic element, wherein the container is preferably formed from the metal layer or comprises the metal layer,

[0044] - 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.

[0045] In hot isostatic pressing, it is particularly intended that the bonding takes place 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 DCB process.

[0046] In comparison to the bonding of a metal layer to a ceramic layer by means of a solder material, which usually uses 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 voids forming 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.

[0047] 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.

[0048] It is preferably provided that during hot isostatic pressing, the metal container is exposed in a heating and pressing device to a gas pressure of between 100 and 2000 bar, preferably between 150 and 1200 bar, and particularly preferably between 300 and 1000 bar, and a process temperature of 300°C up to a melting temperature of the at least one metal layer, in particular up to a temperature below the melting temperature. It has been advantageously 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 bond 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.

[0049] According to a further aspect, a method for producing an electrically insulating ceramic element is provided, comprising:

[0050] - Providing a silicon wafer, preferably a doped or non-doped silicon wafer,

[0051] - Nitriding the silicon wafer to create a silicon nitride layer. All properties and advantages described for the process for producing the metal-ceramic substrate can be applied analogously to the process for producing an electrically insulating ceramic element, and vice versa.

[0052] A further aspect of the present invention is a metal-ceramic substrate produced using the method according to the invention. All properties and advantages described in connection with the method apply analogously to the subject matter of the method, and vice versa.

[0053] In particular, the correspondingly manufactured metal-ceramic substrate is characterized by the fact that the adhesive strengths of the metal layer to the ceramic element and the adhesive strength of the further metal layer to the ceramic element are essentially equal. Furthermore, no residues can be detected on the ceramic element that are attributable to a separating layer, in particular a separating layer comprising boron nitride. Such separating agents are used in the processes known from the prior art, in particular in tape casting processes, to stack several green compacts on top of one another. They cannot be completely removed from the ceramic element and remain on the ceramic element as a sintered skin. Finally, they are still detectable on the manufactured metal-ceramic substrate. This does not apply to the metal-ceramic substrate produced according to the invention.

[0054] 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.

[0055] 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 decreasing active metal content in the bonding layer. A correspondingly high sheet resistance thus corresponds to a low active metal content in the bonding layer.

[0056] 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.

[0057] 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 understood to be its resistance relative to a square surface area. It is customary to designate the surface resistance with the unit OhmZsq(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 value, preferably several measured values, for the thickness are recorded in each of these areas dividing the at least one metal layer. The thicknesses thus determined at different locations are arithmetically averaged.

[0058] 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.

[0059] The bonding layer is preferably formed flatly, in particular without interruption, i.e. continuously, between the at least one metal layer and the ceramic element. It is preferably 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 which are free of metal of the at least one metal layer due to the structuring are not taken into account.

[0060] 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 in the form of a composite made from a silicon disk, in particular a wafer, which is nitrided on the outside. Correspondingly high thermal conductivities also prove to be particularly advantageous for heat dissipation, in particular in addition to the aforementioned surface roughness. This prevents heat from accumulating in the ceramic element after the heat has been quickly dissipated via the interface between the ceramic element and the metal layer. This makes corresponding metal-ceramic substrates particularly advantageous for high-performance electronic components that generate very high levels of heat during operation.

[0061] 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:

[0062] Fig. 1 schematic representation of a metal-ceramic substrate;

[0063] Fig. 2 schematic representation of a method for producing a metal-ceramic substrate according to a first embodiment of the present invention and

[0064] Fig. 3 schematic representation of a method for producing a metal-ceramic substrate according to a second embodiment of the present invention

[0065] 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 to form a metallization.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'.

[0066] The at least one further metal layer 20 serves as a backside 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.

[0067] 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 the thickness of the bonding layer 12 measured in the stacking direction S is comparatively thin. Furthermore, 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 an etching process is provided for the purpose of structuring the at least one metal layer 10. For example, this allows narrower isolation trenches, i.e., distances between individual metal sections of the at least one metal layer 10, to be realized.

[0068] 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.

[0069] 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 the active metal layer and the solder base layer. In this case, the adhesion promoter layer 13 forms part of the bonding layer 12.

[0070] Figure 2 schematically shows 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 in order to provide an insulating ceramic element 30, a silicon wafer 40 is first provided. In particular, this is a silicon wafer 40, i.e. a body made of silicon that runs flat essentially along a main extension plane HSE, particularly preferably a silicon wafer. The silicon wafer 40 can be doped or undoped. The silicon wafer 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.

[0071] Furthermore, it is preferably provided that a first thickness D1 of the silicon wafer 40, measured in a direction running 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 pm, preferably greater than X2 pm, and particularly preferably greater than X3 pm. This advantageously increases the surface area of ​​the silicon wafer 40. Alternatively or additionally, it is conceivable that, in order to increase the surface area of ​​the silicon wafer 40, a profiling is and / or will be formed on the outer side or surface 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 pm, preferably less than Y2 pm, and particularly preferably less than Y3 pm. Furthermore, it is conceivable that the silicon wafer 40 has sections with a first roughness and regions with a second roughness, wherein the first roughness is less than the second roughness. In particular, a surface roughness is modeled along the outer side of the silicon wafer 40. For example, it is also conceivable that the first roughness is formed on a first outer side of the silicon wafer 40 and a second roughness is formed on a second side of the silicon wafer 40 opposite the first side.

[0072] Furthermore, it is conceivable that the silicon wafer 40 is a monocrystalline body or a multicrystalline body. Furthermore, it is conceivable that the grain size of the silicon particles in the silicon wafer 40 is less than 250 pm, preferably less than 100 pm, and particularly preferably less than 50 pm.

[0073] The method, in particular for providing the ceramic element 30, which is subsequently used to form 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 interleaving furnace, 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. The pressure in the gas interleaving furnace is preferably at least 0.2 MPa, preferably at least 0.7 MPa, and particularly preferably at least 0.9 MPa. In particular, a nitrogen atmosphere prevails in the gas interleaving furnace, wherein the nitrogen content should be at least greater than 85%, preferably greater than 90%, and particularly preferably greater than 95%.The conversion efficiency during nitriding is positively influenced by keeping the grain size of the silicon wafer as small as possible. Nitriding occurs, in particular, by forming silicon nitride by diffusing nitrogen into a solid silicon body.

[0074] In the embodiment shown in Figure 2, the entire silicon wafer 40 is not completely nitrided, but rather at least one silicon nitride layer 31 and a further silicon nitride layer 32 are formed on the top and bottom sides 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, wherein the second thickness D2 and / or the third thickness D3 is 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 profilings on the first side and the second side of the silicon wafer 30 opposite the first side, which can influence the conversion efficiency during nitriding. This also leads to different adhesion 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 at least on 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 disk 10, which is structured to form the conductor track and / or connection pads. 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 enables 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. 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 method and material. This ensures the desired symmetry to counteract bending.

[0075] 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 the use of the metal-ceramic substrate 1 as a printed circuit board.

[0076] Figure 3 schematically illustrates a method according to a second exemplary embodiment of the present invention. The embodiment of Figure 3 differs essentially only from the embodiment of Figure 2 in 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. Preferably, the 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 for a partially nitrided silicon wafer to have 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 having silicon in the core. Reference symbols:

[0077] 1 metal-ceramic substrate

[0078] 5 Oven

[0079] 10 metal layer

[0080] 12 binding layer

[0081] 12' additional binding layer

[0082] 13 Adhesion promoter layer

[0083] 20 additional metal layers

[0084] 30 ceramic elements

[0085] 31 silicon nitride layer

[0086] 32 additional silicon nitride layers

[0087] 40 silicon wafers

[0088] HSE main extension level

[0089] S Stacking direction

[0090] D1 first thickness

[0091] D2 second thickness

[0092] D3 third thickness

Claims

Claims 1 . A method for producing a metal-ceramic substrate (1) intended as a printed 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.

2. 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).

3. 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.

4. Method according to one of the preceding claims, wherein the silicon wafer (40) is completely nitrided.

5. Method according to one of the preceding claims, wherein the silicon wafer (40) is monocrystalline.

6. Method according to one of the preceding claims, wherein a grain size associated with silicon grains in the silicon wafer (40) is less than 250 pm, preferably less than 100 pm and particularly preferably less than than 50 m.

7. 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.

8. Method according to one of the preceding claims, wherein the silicon layer (40) has a roughness which is greater than 0.2 pm, preferably greater than 1.0 pm and particularly preferably greater than 2.0 pm.

9. Method according to one of the preceding claims, wherein the silicon layer (40) has a roughness which is less than 1.0 pm, preferably less than 0.5 pm and particularly preferably less than 0.2 pm.

10. Method according to one of the preceding claims, wherein a surface side of the silicon wafer (40) to be nitrided has a surface profiling.

11. A 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).

12. Method according to one of the preceding claims, wherein the silicon wafer (40) is preferably arranged in a furnace, for example with a corresponding holder, such that two opposite sides of the silicon wafer (40) provided for the connection to the metal layer (10) and / or further metal layer are directly exposed to nitrogen.

13. Metal-ceramic substrate (1) produced by a method according to one of claims 1 to 12.

14. Metal-ceramic substrate (1) according to claim 13, wherein the nitrided silicon wafer (40) is substantially free of an accumulation of sintering agents, in particular in the outer circumference of the silicon wafer (40).

15. Metal-ceramic substrate (1) according to claim 13 or 14, wherein an adhesive strength with which the metal layer (10) is bonded to the nitrided silicon wafer (40), in particular to the silicon nitride layer (31), is comparable in magnitude to an adhesive strength with which a further metal layer (20) is bonded to the nitrided silicon wafer (40), in particular to the further silicon nitride layer (32). - 27 - REVISED SHEET (RULE 91) ISA / EP

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