Method for producing cermet substrate and cermet substrate produced by means of such method

Abstract

The invention relates to a method for producing a cermet substrate (1) provided as a circuit board, comprising:-providing a silicon wafer (40), preferably a doped or undoped 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 metallization.

Description

Technical Field

[0001] The invention relates to a method for producing a metal-ceramic substrate and a metal-ceramic substrate produced by means of this method. Background Art

[0002] Metal-ceramic substrates as circuit boards or printed circuit boards are known from the prior art, for example from DE 102013 104739 A1, DE 19 927 046 B4, and DE 10 2009 033 029 A1. Typically, connection areas for electrical components and conductor tracks are provided on the metal-ceramic substrate or on the component side of the metal-ceramic substrate, where the electrical components and conductor tracks can be interconnected to form an electrical circuit. The essential components of a metal-ceramic substrate are an insulating layer, preferably made of ceramic, and at least one metal layer bonded to the insulating layer. Due to their relatively high dielectric strength, ceramic insulating layers have proven particularly advantageous in power electronics. By structuring the metal layer, conductor tracks and / or connection areas for the electrical components can then be realized.

[0003] Silicon nitride ceramics are particularly preferred ceramic elements for forming metal-ceramic substrates, as they have high dielectric strength and high thermal conductivity, which proves to be very advantageous for the long-term use of metal-ceramic substrates configured as circuit boards. For the use of silicon nitride ceramics as insulating elements in metal-ceramic substrates, reference is made, for example, to US Pat. No. 6,242,374 B1.

[0004] Silicon nitride ceramics are typically produced by producing a slurry or slurry-like, planar product from silicon powder or granules, which is then nitrided to convert the silicon into silicon nitride, i.e., in a cast film process. Subsequent sintering transforms the planar product into a solid and stable body that can be used as a ceramic component in a metal-ceramic substrate. Examples of this type of production are described in US 2016 0 362 592 A1, CN 113 248 263 A, and US 10,669,210 B2. Summary of the Invention

[0005] Starting from this prior art, the object of the present application is to provide a metal-ceramic substrate which allows simplified production.

[0006] The invention achieves this object by a method for producing a metal-ceramic substrate according to claim 1, a method according to claim 9, and a metal-ceramic substrate according to claim 10. Further embodiments are apparent from the dependent claims, the description, and the drawings.

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

[0008] - providing a silicon wafer, preferably a doped or undoped 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 provided in this case, which is produced by nitriding a silicon wafer, in particular in the outer region of the silicon wafer. This advantageously makes it possible to use industrially produced silicon wafers that can be provided in large quantities. The nitriding treatment converts the silicon at least partially into silicon nitride, thereby ensuring the desired insulation strength for the nitrided silicon wafers. In particular, it is proposed that the nitriding treatment be carried out in such a way that the produced silicon nitride layer has an insulation strength of 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 ceramic components containing silicon nitride in a cost-effective and simple manner, which are used for insulation in metal-ceramic substrates. The nitrided silicon wafer is used here as a ceramic component. In particular, if the nitrided silicon wafer is not completely nitrided, for example if the core mainly contains silicon, then the present invention also includes the nitrided silicon wafer as a ceramic component. Preferably, the silicon wafer or silicon nitride layer, in particular a completely nitrided one, comprises a proportion of silicon nitride of more than 60% by weight, preferably more than 75% by weight, and particularly preferably more than 90% by weight. It is also conceivable that a partially nitrided silicon wafer, in particular based on the entire ceramic component having silicon in the core, comprises a proportion of silicon nitride of more than 2% by weight, preferably more than 5% by weight, and particularly preferably more than 10% by weight.

[0013] To carry out the nitriding treatment, it is preferably provided that the silicon wafer is placed in a furnace, in particular a gas sintering furnace, and placed in a nitrogen atmosphere. It is particularly preferred that a pressure of at least 0.2 MPa, preferably greater than 0.5 MPa, and particularly preferably greater than 0.8 MPa prevails or exists in the furnace. It is also preferred that the silicon wafer be placed in the nitrogen atmosphere for at least between 0.5 and 10 hours, preferably between 1 and 5 hours, and particularly preferably between 1.5 and 3 hours. Temperatures of between 1000°C and 2000°C, preferably between 1200°C and 1800°C, and particularly preferably between 1300°C and 1500°C, can preferably be present. It has also been shown that the nitriding treatment temperatures do not impair electrical conductivity.

[0014] The nitriding process converts at least a portion of the silicon into silicon nitride, thereby forming a layer that can be used as a ceramic component in a metal-ceramic substrate with an insulating effect. The entire silicon wafer, which may only be partially nitrided, is then considered a ceramic component for a metal-ceramic substrate, even if the nitrided silicon nitride wafer includes subregions that are not purely ceramic from a technical point of view.

[0015] The silicon wafer is preferably heated in multiple stages, at least in two stages, to avoid melting of the silicon wafer due to high temperatures, whereby α-silicon nitride is preferably formed first, which is then converted into β-silicon nitride in a second temperature stage.

[0016] In particular, it is proposed that the final conversion of silicon preferably forms only β-silicon nitride, for example, the 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.

[0017] Furthermore, it is particularly preferred that the grain size associated with the silicon grains is less than 250 μm, preferably less than 100 μm, and particularly preferably less than 50 μm. It has been shown that the conversion efficiency during the nitridation process increases with decreasing grain size. This allows the efficiency of the conversion of silicon into silicon nitride to be correspondingly increased.

[0018] The silicon wafer is preferably a single crystal or a monocrystal or a polycrystal, which is particularly preferably of planar design and extends along a main extension plane. In particular, this is a silicon wafer. In particular, the nitrided silicon wafer is also of single-crystalline design. Preferably, the silicon wafer is not a green body, i.e., the silicon wafer does not have a paste-like consistency, but is to be understood as a self-supporting body that does not deform when simply lifted from one side, but rather remains essentially dimensionally stable. In particular, the chemical and physical properties of the single-crystal silicon wafer differ from those of silicon wafers produced by pressing and / or sintering silicon powder to form the silicon wafer. It is preferably provided that at least a portion of the silicon is converted into silicon nitride in order 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 film casting method.

[0019] Preferably, the metal-ceramic substrate is designed as a circuit board, and in the finished state, at least one metal layer bonded to the ceramic element is structured. For example, it is proposed that, after the bonding step, structuring is further performed, for example by laser, etching, and / or mechanical processing, with the aid of which printed conductors and / or interfaces for the electrical or electronic component are realized. It is preferably proposed that a further metal layer, in particular a back metallization, and / or a cooling element is provided on the finished metal-ceramic substrate and on the ceramic element on the side opposite the metal layer. The back metallization preferably serves to resist bending, and the cooling element serves to effectively dissipate heat generated during operation of the electrical or electronic component bonded to the circuit board or metal-ceramic substrate.

[0020] As materials for at least one metal layer and / or at least one further metal layer in the metal-ceramic substrate, copper, aluminum, molybdenum, tungsten, nickel and / or alloys thereof, such as CuZr, AlSi or AlMgSi, as well as laminates such as CuW, CuMo, CuAl and / or AlCu or MMCs (metal matrix composites) such as CuW, CuM or AlSiC, can be envisaged. Furthermore, it is preferably provided that at least one metal layer on the finished metal-ceramic substrate, in particular as a component metallization, is surface-modified. As surface modifications, for example, sealing with precious metals, in particular silver and / or gold, or (electroless) nickel or ENIG (electroless nickel immersion gold), or edge sealing at the metallization, can be considered to suppress crack formation or widening.

[0021] It is preferably proposed that a further silicon nitride layer is formed on the silicon wafer during the nitriding treatment on the side opposite to at least one silicon wafer. This advantageously makes it possible to form a silicon nitride layer on the opposite side, which allows the metal layer and the further metal layer to be bonded to the opposite side. In this way, the metal layer and the further metal layer serving as the back metallization in the finished metal-ceramic substrate can be bonded to the ceramic element. The back metallization serves in particular to make the top and bottom of the metal-ceramic substrate symmetrical in order to thus resist bending. Due to the common design of the silicon nitride layer, in particular the same active solder material can be used, so that in a common working step the metal layer and the further metal layer can be bonded to the nitrided silicon wafer, which serves as a ceramic element for the finished substrate.

[0022] It is preferably proposed that the adhesion strength of the metal layer to the ceramic element, in particular to the silicon nitride layer, is of similar magnitude to the adhesion strength of the other metal layer to the ceramic element, in particular to the other silicon nitride layer. For example, the adhesion strengths differ from each other in their measurable magnitude by no more than 5%, preferably by no more than 2.5% and particularly preferably by no more than 1%. This achieves adhesion strengths that are as symmetrical as possible on the device side and back side of the metal-ceramic substrate. This is achieved in the following way: if the top and bottom sides of the nitrided silicon nitride wafer are subjected to nitrogen to the same extent during the nitriding process, then the top and bottom sides have the same surface specifications. To this end, the silicon wafer is preferably arranged in a furnace, for example by means of corresponding holders, so that the two opposite sides of the silicon wafer, which are provided for bonding to the metal layer and / or the other metal layer, are directly subjected to nitrogen. In this case, the opposite sides extend in particular along a plane extending parallel to the main extension plane. As a result, in contrast to silicon nitride ceramics according to the prior art, which are placed on the processing surface as a slurry during production and are difficult to come into contact with nitrogen on one side. In such ceramic components produced according to the prior art, the surface specifications on the opposite sides therefore differ from the surface specifications of the silicon wafer which has been nitrided on both sides.

[0023] Furthermore, the resulting metal-ceramic substrate differs from the metal-ceramic substrates known from the prior art in that the ceramic element, i.e., the nitrided silicon wafer, has essentially no accumulation of sintering agent around its outer circumference. In ceramic elements formed by film casting, a layer approximately 20 to 30 μm thick is ultimately formed, in which sintering agent, which diffused there during the sintering process, accumulates. This is referred to as a so-called sintering skin. Examples of sintering agents are, for example, MgO or Y2O3. In contrast, the sintering agent in the silicon nitride layer is essentially uniformly distributed throughout the nitrided silicon wafer.

[0024] It is preferably provided that the silicon wafer has a first thickness, wherein at least one silicon nitride layer has a second thickness and further silicon nitride layers have a third thickness, wherein the ratio of the second thickness and / or the third thickness to the first thickness is less than 0.5, preferably less than 0.2 and particularly preferably less than 0.1. This means that it is not necessary to completely nitridate the silicon wafer; it is sufficient to nitridate the outermost circumference of the silicon wafer, preferably circumferentially, and to initially form the silicon nitride layer and / or further silicon nitride layer 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, extending both on the upper side and on the lower side of the silicon nitride wafer, i.e., on opposite sides of the silicon nitride wafer. The second thickness and / or the third thickness are preferably set so as to provide a sufficient dielectric strength of the silicon nitride layer for the application.

[0025] In particular, a soft core is produced between the silicon nitride layer and the further silicon nitride layer, in the intermediate region between the device metallization and the back metallization. It is also conceivable that the core region between the silicon nitride layer and the further silicon nitride layer is given semiconductor properties by doping, for example, with boron, phosphorus and / or nitrogen. Furthermore, it has advantageously been found that the temperature resistance of the resulting metal-ceramic substrate can be specifically influenced by setting the ratio between the first thickness, the second thickness and the third thickness. Furthermore, the non-nitrided soft 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.

[0026] It is preferably provided that the silicon nitride layer is completely nitrided. Partial nitriding of the silicon wafer has proven to be particularly advantageous if sufficient dielectric strength must be established as quickly as possible. This shortens the work process, in particular the nitriding process, and also reduces the effort involved in producing the nitrided silicon wafer that can be used as a ceramic component. Complete nitriding has proven to be particularly advantageous if relatively thin silicon wafers are available, as complete nitriding is already possible in this case to ensure dielectric strength, or if high dielectric strength is required.

[0027] 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 a silicon wafer that can be used directly as a ceramic element in a metal-ceramic substrate.

[0028] Furthermore, it is proposed that the silicon layer has a roughness 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 of the silicon wafer, thereby increasing the conversion efficiency during the nitriding process.

[0029] It is particularly preferred that the silicon layer have a roughness of less than 1.0 μm, preferably less than 0.5 μm, and particularly preferably less than 0.2 μm. This provides a particularly smooth, in particular flat-sawn surface that is nitrided. This proves to be particularly advantageous when no further work steps are required to process the silicon wafer.

[0030] Furthermore, it is proposed 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 recesses, for example with a triangular or square cross-sectional shape. This allows the surface to be specifically adapted to the optimal framework conditions for the nitriding process. Furthermore, if the nitrided silicon wafer is used to bond a metal layer thereto, the recessed portion can be used to collect active solder material.

[0031] In order to produce the desired roughness and / or contouring in the silicon wafer, it is preferably provided that a preparatory step is provided in which the surface or the outer side of the silicon wafer is roughened and / or contoured at least in sections before nitriding in order to increase the surface area. This provides a larger area that can be used for subsequently bonding the metal layer to the ceramic component.

[0032] It is preferably provided that, to form the metal-ceramic substrate, the metal layer is bonded to the silicon nitride layer by means of an active soldering method and / or hot isostatic pressing. This ensures a durable bond of the metal layer to the silicon nitride layer, which is particularly advantageous for printed circuit boards. It is also preferably provided that the metal-ceramic substrate has a metallization, i.e., a structured metal layer in which the individual metal sections are electrically insulated from one another.

[0033] For example, it is proposed to propose a method for producing a metal-ceramic substrate, which method comprises: providing a solder layer, in particular in the form of at least one solder film or hard solder film,

[0034] coating a ceramic element, in particular a nitrided silicon wafer, and / or at least one metal layer and / or at least one solder layer with at least one active metal layer,

[0035] - arranging at least one solder layer between the ceramic element and the at least one metal layer in the stacking direction to form a solder system, the solder system comprising at least one solder layer and at least one active metal layer, wherein the solder material of the at least one solder layer preferably contains no material with a low melting point or no phosphorus-containing material, and

[0036] - bonding the at least one metal layer to the at least one ceramic layer via a soldering system by means of an active solder method.

[0037] In particular, a multilayer welding system is provided, comprising at least one welding layer, preferably one without melting point-depressing elements, particularly preferably a phosphorus-free welding layer, and at least one active metal layer. The separation of the at least one active metal layer and the at least one welding layer has proven particularly advantageous, as it allows for relatively thin welding layers, particularly when the welding layer is a film. Otherwise, for welding materials containing active metals, relatively large welding layer thicknesses must be achieved due to the common brittle intermetallic phases or high E modulus and high yield strength of active metals, as well as their intermetallic phases that hinder deformation of the solder paste or welding layer. Consequently, the minimum layer thickness is limited by the production characteristics of the active metal-containing welding material. Accordingly, for welding layers containing active metals, the minimum welding layer thickness is determined not by the minimum thickness required for the joining method, but by the minimum thickness technically achievable. Consequently, such thicker active metal-containing welding layers are more expensive than thin layers. Those skilled in the art particularly understand "phosphorus-free" to mean a phosphorus content in the welding layer of less than 150 ppm, less than 100 ppm, and particularly preferably less than 50 ppm.

[0038] In particular, by using a separately formed active metal layer, it is possible to design the active metal layer to be relatively thin, thereby achieving a relatively thin thickness of the bonding layer as required, in particular by averaging different measured values ​​within a fixed 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 active metals are titanium (Ti), zirconium (Zr), hafnium (Hf), chromium (Cr), niobium (Nb), cerium (Ce), tantalum (Ta), magnesium (Mg), lanthanum (La), and vanadium (V). In this context, it should be noted that the metals La, Ce, Ca, and Mg are susceptible to oxidation. Furthermore, it should be noted that the elements Cr, Mo, and W are not conventional active metals, but are suitable as contact layers between Si3N4 and at least one metal layer or soldering system or solder material because they do not form intermetallic phases with the at least one metal layer, such as copper, and do not have edge solubility.

[0039] Preferably, the proportion of active metal in the active metal layer or in the adhesion promoter layer comprising active metal is greater than 15% by weight, preferably greater than 20% by weight and particularly preferably greater than 25% by weight.

[0040] Preferably, the solder layer, in particular a phosphorus-free solder layer, contains a variety of materials in addition to pure metal. For example, indium is a component of the solder material used in the solder layer. The solder layer or solder base material preferably contains less than 1.5% by weight, preferably less than 1.0% by weight, and particularly preferably less than 0.5% by weight of active metals. In particular, the solder base material contains no active metals.

[0041] 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 electroplating. This advantageously makes it possible to achieve a relatively thin solder layer, in particular with uniform distribution, in the soldering system.

[0042] For example, when manufacturing a metal-ceramic substrate, in particular a metal-ceramic substrate, further steps are provided, including:

[0043] - providing a ceramic element and a metal layer,

[0044] - providing a gas-tight container which surrounds the ceramic element, wherein the container is preferably formed by or comprises a metal layer,

[0045] - forming a cermet substrate by bonding a metal layer to a ceramic element by means of hot isostatic pressing,

[0046] To form the metal-ceramic substrate, an active metal layer or a contact layer containing an active metal is provided at least in sections between the metal layer and the ceramic element to facilitate bonding of the metal layer to the ceramic element. The container is preferably a metal container composed of the metal layer and / or further metal layers. Alternatively, a glass container is also conceivable.

[0047] In hot isostatic pressing, it is particularly advantageous to perform bonding by heating under pressure, without the first and / or second metal layer of the metal container, in particular the subsequent metal layer of the cermet substrate, and any eutectic layer present therein, being transformed into a molten phase. Accordingly, lower temperatures are required in hot isostatic pressing than in direct metal bonding methods, in particular the DCB method.

[0048] Compared to bonding a metal layer to a ceramic layer using a solder material (where temperatures below the melting temperature of at least one metal layer are typically used), this process advantageously dispenses with a solder base material and only requires reactive metal. Furthermore, the use or utilization of pressure during hot isostatic pressing has proven advantageous, as it reduces air inclusions or cavities between the first and / or second metal layer on the one hand and the ceramic element on the other hand, thereby reducing or even preventing the formation of pores in the resulting or manufactured metal-ceramic substrate. This has a positive impact on the bond quality between the metal layer or the first and / or second metal layer of the metal container and the ceramic element. Furthermore, it is advantageously possible to simplify "second etching" and avoid solder residues and silver migration.

[0049] 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 the 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.

[0050] It is preferably provided that during hot isostatic pressing, the metal container is subjected in a heating and pressure device to a gas pressure of between 100 bar and 2000 bar, preferably between 150 bar and 1200 bar, and particularly preferably between 300 bar and 1000 bar, and a process temperature of 300° C. up to the melting temperature of at least one metal layer, in particular to a temperature below the melting temperature. Advantageously, it has been shown that the metal layer, i.e., the first and / or second metal layer of the metal container, can be bonded to the ceramic component without requiring the temperatures required for direct metal bonding methods such as DCB or DAB, and / or without requiring a solder base material used in active soldering. Furthermore, utilizing or using a corresponding gas pressure allows the production of a metal-ceramic substrate that is largely free of pores, i.e., gas inclusions, between the metal layer and the ceramic component. In particular, the process parameters mentioned in DE 2013 113 734 A1 are used, to which reference is expressly made.

[0051] According to another aspect, a method for producing an electrically insulating ceramic component is provided, the method comprising:

[0052] - providing a silicon wafer, preferably a doped or undoped silicon wafer,

[0053] Nitriding the silicon wafer to produce a silicon nitride layer. All properties and advantages described with respect to the method for producing a metal-ceramic substrate can be transferred analogously to the method for producing an electrically insulating ceramic component, and vice versa.

[0054] Another aspect of the present invention is a metal-ceramic substrate produced by means of the method according to the invention. All properties and advantages described in conjunction with the method apply analogously to the subject matter of the method, and vice versa.

[0055] In particular, the metal-ceramic substrates manufactured accordingly are characterized in that the adhesion strength of the metal layer on the ceramic element and the adhesion strength of the other metal layer on the ceramic element are substantially the same. In addition, no residues are visible on the ceramic element, which residues are attributed to the separation layer, in particular the separation layer containing boron nitride. In methods known from the prior art, in particular in the film casting method, such separating agents are used to stack multiple green compacts one above the other. The separating agents cannot be removed from the ceramic element without residue, but remain on the ceramic element as a sintered skin. Ultimately, they are still detectable on the manufactured metal-ceramic substrate. This is not applicable to the metal-ceramic substrate manufactured according to the present invention.

[0056] According to a preferred embodiment of the present invention, a bonding layer is formed between the metal layer and the ceramic element, in particular the silicon nitride layer, in the manufactured metal-ceramic substrate, wherein the adhesion promoter layer of the bonding layer has a surface resistance greater than 5 ohms / square, preferably greater than 10 ohms / square and particularly preferably greater than 20 ohms.

[0057] The surface resistance is directly related to the active metal content in the adhesion promoter layer, which primarily serves to bond the at least one metal layer to the ceramic component. As the active metal content in the bonding layer decreases, the surface resistance increases. Therefore, a correspondingly high surface resistance corresponds to a low active metal content in the adhesion promoter layer.

[0058] Surface resistance is not dependent on a single parameter but can be influenced by the interaction of multiple 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 resistance can only be achieved through the interaction of at least two parameters.

[0059] It has been found that increasing the proportion of active metal promotes the formation of brittle intermetallic phases, which in turn impairs the peel strength of the metal layer at the insulating layer. In other words, the required surface resistance describes a bonding layer whose peel strength is improved, i.e., increased, due to the reduced formation of brittle intermetallic phases. Therefore, by specifically setting the required surface resistance, a particularly strong bond of the at least one metal layer to the ceramic component can be achieved. This increased bond strength has a beneficial effect on the service life of the cermet substrate. To determine the surface resistance, it is proposed that the metal layer and, optionally, the solder base layer be removed again, for example, by etching, from the finished cermet substrate. The surface resistance is then measured on the outer side or underside of the cermet substrate, from which the at least one metal layer and the solder base layer have been removed, using a four-point measurement. In particular, the surface resistance of a material sample is understood to be its resistance per square surface area. In this context, the surface resistance is typically expressed in the unit "ohm / square." The physical unit of surface resistance is the ohm. Preferably, the thickness of the bonding layer, measured in the stacking direction, is averaged over a plurality of measuring points within a predetermined surface or surfaces extending parallel to the main extension plane to a value of less than 0.20 mm, preferably less than 10 μm, and particularly preferably less than 6 μm. The term "multiple surfaces" particularly refers to dividing the at least one metal layer into surfaces of as equal size as possible and detecting at least one value, preferably multiple measured values, for the thickness determination in each surface dividing the at least one metal layer. The thicknesses thus determined at different locations are arithmetically averaged.

[0060] Therefore, compared with the metal ceramic substrates known in the prior art, a relatively thin bonding layer is formed between the at least one metal layer and the ceramic element. It is proposed here that, in order to determine the critical thickness of the bonding layer, the thickness measured via a plurality of measuring points in one or more predetermined or fixed surfaces is averaged. This is advantageously taken into account: the ceramic element is usually subject to fluctuations, i.e. the waviness is transmitted to the ceramic element. In particular, a person skilled in the art understands waviness as a change in the overall flat direction of the ceramic element when viewed along a direction extending parallel to the main extension plane by several millimeters or centimeters. Thus, such fluctuations are different from the surface roughness of the ceramic element that is usually additionally present at the ceramic element. By incorporating such generally unavoidable fluctuations of the ceramic element into the thickness determination, it is taken into account that the bonding layer may change due to the fluctuations, in particular it may be greater in the recessed areas of the ceramic element than in the raised areas of the ceramic element.

[0061] Preferably, the bonding layer is formed planarly, in particular without interruptions, i.e., continuously, between the at least one metal layer and the ceramic element. It is preferably provided that the ratio of the area between the at least one metal layer and the ceramic element that does not form a bonding layer to the area between the at least one bonding layer and the ceramic element that does form a bonding layer is less than 0.05 mm, preferably less than 0.02 mm, and particularly preferably less than 0.007 mm. A person skilled in the art will particularly appreciate that areas that do not contain metal of the at least one metal layer due to structuring are not taken into account for calculating this ratio.

[0062] 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 component consisting of silicon wafers, in particular wafers, which are nitrided from the outside. Furthermore, a correspondingly high thermal conductivity proves to be particularly advantageous for heat dissipation, in particular in addition to the above-mentioned surface roughness. This prevents heat from accumulating in the ceramic element after rapid heat dissipation via the interface between the ceramic element and the metal layer. Consequently, corresponding metal-ceramic substrates can be used particularly advantageously for high-performance electronic components that generate very high amounts of heat during operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Further advantages and features emerge from the following description of preferred embodiments of the subject matter according to the invention with reference to the accompanying drawings. The drawings show:

[0064] Figure 1 A schematic diagram showing a metal-ceramic substrate;

[0065] Figure 2 a schematic diagram showing a method for manufacturing a metal-ceramic substrate according to a first embodiment of the present invention; and

[0066] Figure 3 A schematic diagram shows a method for producing a metal-ceramic substrate according to a second embodiment of the present invention. DETAILED DESCRIPTION

[0067] exist Figure 1 shows a metal-ceramic substrate 1 according to an exemplary first embodiment of the present invention. This metal-ceramic substrate 1 is preferably used as a carrier or circuit board for an electronic or electrical component that can be bonded to 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 to form corresponding conductor tracks and / or connection surfaces. In other words, in the finished metal-ceramic substrate 1, the at least one metal layer 10 comprises a plurality of electrically insulated metal segments to form a metallization. The at least one metal layer 10, which extends substantially along a main plane of extension HSE, and the ceramic element 30, which extends along the main plane of extension HSE, are arranged one above the other along a stacking direction S extending perpendicularly to the main plane of extension HSE and are preferably joined or connected to one another via a bonding layer 12. Preferably, in addition to the at least one metal layer 10, the metal-ceramic substrate 1 includes at least one further metal layer 20, which is arranged on the side of the ceramic element 30 opposite the at least one metal layer 10, as viewed in the stacking direction S, and is bonded to the ceramic element 30 via a further bonding layer 12′.

[0068] In this case, the at least one further metal layer 20 serves as a back metallization which counteracts bending of the metal-ceramic substrate 1 and / or as a heat sink designed to dissipate heat input caused by electrical or electronic components on the metal-ceramic substrate 1 .

[0069] 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 that the thickness of the bonding layer 12, measured in the stacking direction S, is relatively thin. Furthermore, if an etching process is provided for structuring the at least one metal layer 10, a relatively thin thickness of the bonding layer 12 between the at least one metal layer 10 and the ceramic element 30 has proven advantageous. For example, this allows for narrower insulation trenches, i.e., spacings, between the individual metal sections of the at least one metal layer 10.

[0070] Furthermore, forming the bonding layer 12 thinner has proven to be advantageous, since the number of possible defects in the bonding layer 12 due to material defects in the solder material that may be used can thereby also be further reduced.

[0071] exist Figure 1In the example shown in FIG, the bonding layer 12 comprises, in particular, an adhesion promoter layer 13 of an active metal. In this case, after bonding, the adhesion promoter layer 13 is preferably formed from a material composition comprising, on the one hand, a compound consisting of a component of the ceramic element and, on the other hand, an active metal. Since this involves compounds that fracture very brittlely, a design of these adhesion promoter layers 13 that is as thin as possible is advantageous with regard to the adhesion strength of the at least one metal layer 10 on the ceramic element 30. For example, if, for example, an active metal layer, in particular an active metal film, is provided for the bonding process between the ceramic element 30 and the metal layer 10, and the bonding process is carried out via hot isostatic pressing, then the adhesion promoter layer 13 can form the bonding layer 12. However, the adhesion promoter layer 13 can also be formed, for example, by an active metal layer, in particular an active metal film, which is provided between the ceramic element 30 and the welding base layer in order to produce a bond between the metal layer 10 and the ceramic element 30 via a system consisting of an active metal layer and a welding base layer. In this case, the adhesion promoter layer 13 forms part of the bonding layer 12.

[0072] exist Figure 2 Schematically illustrates the production of a metal-ceramic substrate 1 for use as a circuit board according to a first exemplary embodiment. In particular, it is provided that a silicon wafer 40 is first provided to provide the insulating ceramic element 30. In particular, this is a silicon wafer 40, i.e., a body composed of silicon extending substantially along a main plane of extension HSE, particularly preferably a silicon wafer. Silicon wafer 40 can be doped or undoped. Preferably, silicon wafer 40 has a silicon content of greater than 50% by weight, preferably greater than 70% by weight, and particularly preferably greater than 80% by weight.

[0073] Furthermore, it is preferably provided that the first thickness D1 of the silicon wafer 40, measured in a direction extending perpendicularly to the main plane of extension 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 according to standards and is preferably round. It is conceivable that the outer side, in particular the surface, of the silicon wafer 40 has a roughness greater than X1 μm, preferably greater than X2 μm, and particularly preferably greater than X3 μm.

[0074] This advantageously improves the surface of the silicon wafer 40. Alternatively or in addition, it is conceivable that, in order to improve the surface of the silicon wafer 40, a contour is already formed and / or 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 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 areas with a first roughness and areas with a second roughness, wherein the first roughness is less than the second roughness. In particular, the surface roughness along the outer side of the silicon wafer 40 is modeled. For example, it is also conceivable that a 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.

[0075] It is also conceivable that the silicon wafer 40 is monocrystalline or polycrystalline. Furthermore, it is conceivable that the grain size of the silicon particles in the silicon wafer 40 is less than 250 μm, preferably less than 100 μm and particularly preferably less than 50 μm.

[0076] In particular, to provide the method for subsequently forming a ceramic component 30 for a metal-ceramic substrate 1, a second method step involves at least partially nitriding the silicon wafer 40. To this end, the silicon wafer 40 is placed in a furnace 5, particularly a gas sintering furnace, for nitriding there. The silicon wafer 30 is subjected to temperatures between 1000°C and 2000°C, preferably between 1100°C and 1800°C, and particularly preferably between 1300°C and 1500°C. A pressure of at least 0.2 MPa, preferably at least 0.7 MPa, and particularly preferably at least 0.9 MPa, prevails in the gas sintering furnace. In particular, a nitrogen atmosphere prevails in the gas sintering 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 the nitriding process is favorably influenced by the smallest possible grain size in the silicon wafer. The nitriding process is particularly performed so that silicon nitride forms by diffusion of nitrogen into the solid silicon body.

[0077] exist Figure 2In the embodiment shown in FIG, the entire silicon wafer 40 is not completely nitrided. Instead, at least one silicon nitride layer 31 and a further silicon nitride layer 32 are formed on the upper and lower sides of the silicon wafer 40, i.e., on opposite sides of the silicon wafer 40. Here, 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 are less than the first thickness D1 of the silicon wafer 30. This indicates that a complete nitridation process is not performed, i.e., the entire silicon wafer 30 is not completely nitrided. In particular, the nitridation process is performed to form a silicon nitride layer 31 having a sufficient dielectric strength for use of 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 roughness and / or contours on a first side of the silicon wafer 30 and on a second side opposite the first side, thereby influencing the conversion efficiency during the nitriding process. This also results in different adhesion strengths of the metal layer and the further metal layer on opposite sides of the ceramic element. It is also conceivable that the silicon wafer 40 is placed on a covering surface in the furnace so that only a reduced nitriding process is performed on at least one side. In this case, essentially only at least one silicon nitride layer 31 is formed, which faces the at least one metal sheet 10 in the ceramic-metal substrate 2 and is structured to form conductor tracks and / or connection surfaces.

[0078] After the nitriding treatment, at least one metal layer 10 is bonded to at least one silicon nitride layer 31, preferably using an active soldering method and / or hot isostatic pressing. This advantageously allows the metal layer 10 to be applied to the ceramic component 30, in particular the silicon nitride layer 31. It is preferably provided that, 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 that the metal layer 10 and the 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 materials. This ensures the desired symmetry to counteract bending.

[0079] After at least one metal layer 10 or at least one further metal layer 20 has been bonded to a nitrided silicon wafer serving as a ceramic element 30, the at least one metal layer is preferably structured in order to provide connection surfaces and / or conductor tracks which are provided for using the metal-ceramic substrate 1 as a circuit board.

[0080] exist Figure 3 The method according to the exemplary second embodiment of the present invention is schematically shown in FIG. Figure 3 Examples and Figure 2 The embodiment differs essentially only in that the silicon wafer 40 is completely nitrided. 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 that the partially nitrided silicon wafer has a proportion of silicon nitride greater than 2% by weight, preferably greater than 5% by weight, and particularly preferably greater than 10% by weight, based on the entire ceramic component having silicon in the core.

[0081] Reference Signs List

[0082] 1 Metal-ceramic substrate

[0083] 5 furnaces

[0084] 10 Metal Layer

[0085] 12. Bonding layer

[0086] 12' Additional bonding layer

[0087] 13 Adhesion promoter layer

[0088] 20 additional metal layers

[0089] 30 ceramic components

[0090] 31 silicon nitride layer

[0091] 32 Additional silicon nitride layer

[0092] 40 silicon wafers

[0093] HSE Main Extension Plane

[0094] S Stacking direction

[0095] D1 first thickness

[0096] D2 Second thickness

[0097] D3 third thickness

Claims

1. A method for manufacturing a metal-ceramic substrate (1) configured as a circuit board, the method comprising: - providing a silicon wafer (40), preferably a doped or undoped silicon wafer (40), - performing a nitridation process on the silicon wafer (40) to produce a silicon nitride layer (31), - bonding the metal layer (10) to the silicon nitride layer (31), and - Structuring the metal layer (10) to form a metallization.

2. The method according to claim 1, wherein a further silicon nitride layer (32) is formed during the nitridation treatment on the silicon wafer (40) on the side opposite the silicon nitride layer (31).

3. A method according to any of the above 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 the 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. The method according to any one of the preceding claims, wherein the silicon wafer (40) is completely nitrided.

5. The method according to any one of the preceding claims, wherein the silicon wafer (40) is single crystalline.

6. The method according to any of the preceding claims, wherein the grain size associated with the silicon grains in the silicon wafer (40) is less than 250 μm, preferably less than 100 μm and particularly preferably less than 50 μm.

7. The method according to any 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. The method according to any of the preceding claims, wherein the silicon layer (40) has a roughness greater than 0.2 μm, preferably greater than 1.0 μm and particularly preferably greater than 2.0 μm.

9. The method according to claim 1, wherein the silicon layer (40) has a roughness of less than 1.0 μm, preferably less than 0.5 μm and particularly preferably less than 0.2 μm.

10. The method according to claim 1, wherein the surface side of the silicon wafer (40) to be nitrided has a surface profiling.

11. A method for producing an electrically insulating ceramic component, the method comprising: - providing a silicon wafer (40), preferably a doped or undoped silicon wafer (40), - subjecting the silicon wafer (40) to a nitridation process to produce a silicon nitride layer (31).

12. A method according to any of the preceding claims, wherein the silicon wafer (40) is preferably arranged in a furnace, for example by means of corresponding retaining members, so that the opposite sides of the silicon wafer (40) arranged for bonding to the metal layer (10) and / or another metal layer are directly exposed to nitrogen.

13. A metal-ceramic substrate (1) produced by means of a method according to any one of claims 1 to 12.

14. The metal-ceramic substrate (1) according to claim 13, wherein the nitrided silicon wafer (40) has substantially no accumulation of sintering agent, in particular, no accumulation of sintering agent in the outer circumference of the silicon wafer (40).

15. A metal-ceramic substrate (1) according to claim 13 or 14, wherein the adhesion strength of the metal layer (10) to the nitrided silicon wafer (40), in particular to the silicon nitride layer (31), is similar in magnitude to the adhesion strength of another metal layer (20) to the nitrided silicon wafer (40), in particular to the another silicon nitride layer (32).

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