METHOD FOR PRODUCING A METAL-CERAMIC SUBSTRATE

Description

[0001] The present invention relates to a method for producing a metal-ceramic substrate and a metal-ceramic substrate.

[0002] Metal-ceramic substrates play a crucial role in power electronics. They are a key element in the construction of electronic components and ensure the rapid dissipation of large amounts of heat during operation. Metal-ceramic substrates typically consist of a ceramic layer and a metal layer bonded to the ceramic layer.

[0003] Several methods for bonding the metal layer to the ceramic layer are known in the prior art. In the so-called DCB ("Direct Copper Bonding") process, a copper foil is surface-coated with a copper compound (usually copper oxide) that has a lower melting point than copper by reacting copper with a reactive gas (usually oxygen). When the copper foil treated in this way is applied to a ceramic body and the composite is heated, the copper compound melts and wets the surface of the ceramic body, resulting in a stable, metallurgical bond between the copper foil and the ceramic body. This process is described, for example, in US 3744120 A and DE 2319854 C2.

[0004] In an alternative method, metal foils can be joined to ceramic bodies at temperatures of approximately 650 to 1000°C using a special solder containing a metal with a melting point of at least 700°C (usually silver) and an active metal. The role of the active metal is to react with the ceramic material, thus enabling the ceramic material to bond with the remaining solder to form a reaction layer, while the metal with a melting point of at least 700°C serves to bond this reaction layer to the metal foil. For example, JP4812985 B2 proposes joining a copper foil to a ceramic body using a solder containing 50 to 89 percent by weight of silver, as well as copper, bismuth, and an active metal. This method reliably joins the copper foil to the ceramic body.To avoid problems related to silver migration, it can be advantageous to use silver-free solders for joining metal foils to ceramic bodies. These solders are based, for example, on high-melting-point metals (especially copper), low-melting-point metals (such as bismuth, indium, or tin), and active metals (such as titanium). Such a technique is proposed, for example, in DE 102017114893 A1.

[0005] In the assembly of electronic components, metal-ceramic substrates are typically populated with a component, especially a chip. For component placement on the metal-ceramic substrate, it is generally necessary that the area of ​​the substrate to be populated with the component has a silver-containing contact area. Providing this silver-containing contact area facilitates the connection of the component to the metal-ceramic substrate using common methods such as sintering or soldering. To create the contact area, the metal-ceramic substrate is usually first treated in certain areas with an etching solution to form the desired structure. The contact area is then created by applying a silver-containing coating to the surface of the structured metal-ceramic substrate in these areas.This is usually done by electrochemical deposition of silver.

[0006] A disadvantage of electrochemical deposition is that it damages the surface of the metal-ceramic substrate (both in and around the contact area). This often results in the formation of holes in the metal surface of the metal-ceramic substrate, which can create defects for bonded components, particularly electronic components or contact elements (such as bond wires). Consequently, less surface area is available for bonding these components. This can lead to a situation where, although bonding of a component to the metal-ceramic substrate is possible, the adhesive strength is reduced. Furthermore, electrochemical deposition also attacks the metal layer of the metal-ceramic substrate, especially at its interface with the ceramic. In this process, metal ions are electrochemically leached from the metal layer of the metal-ceramic substrate and replaced by silver ions.In this process, silver ions are predominantly deposited on the surface of the metal-ceramic substrate, while the metal ions of the metal layer are predominantly dissolved in an area near the ceramic body. This leads to holes in the bonding area and consequently to a decrease in the reliability of the metal-ceramic substrate, which is particularly evident under temperature cycling conditions when the metal layer detaches from the ceramic body.

[0007] A further disadvantage of the methods known from the prior art is that the selective deposition of silver by electrochemical processes requires that the areas on the surface of the metal-ceramic substrate, where no silver is to be deposited, be protected. For this purpose, these areas are covered with a mask, which must be removed after the silver deposition. However, masking these areas is technically complex and, moreover, it cannot prevent the formation of holes in the bonding area, as the silver-containing solution undermines the mask due to edge-level structuring in the metal layer. Furthermore, masking residues typically remain on the surface, which can impair the function of the metal-ceramic substrate.

[0008] From US 2021 / 136963 A1, a method for manufacturing an electronic module is known in which at least two electronic components are attached to a substrate, these are covered with a dielectric material in such a way that a recess is created, and an ink is printed onto a first surface of the recess, which contains a metal precursor and an organic compound that is liquid at room temperature and ambient pressure.

[0009] EP 2814788 A1 comprises a process for producing a metal-ceramic substrate in which a first surface side of the ceramic is joined with a first metallization and a second surface side of the ceramic is joined with a second metallization, wherein the first metallization is produced using a DCB process, while the second metallization is produced from aluminum or an aluminum alloy.

[0010] It would therefore be desirable to provide a method for producing a metal-ceramic substrate having a silver-containing contact area that overcomes at least one disadvantage of the prior art methods.

[0011] An object of the present invention is therefore preferably to provide a method for producing a metal-ceramic substrate comprising a silver-containing contact area, with which a metal-ceramic substrate can be obtained with which components to be bonded, in particular components and contacting means (such as bonding wires), can be connected with high adhesive strength.

[0012] A further object of the present invention is preferably to provide a method for producing a metal-ceramic substrate comprising a silver-containing contact area, with which a metal-ceramic substrate can be obtained that exhibits high reliability under temperature cycling stresses (in the sense of a stable connection between the metal layer and the ceramic body).

[0013] A further object of the present invention is preferably to provide a method for producing a metal-ceramic substrate having a silver-containing contact area, in which masking of the surface of the metal layer to create the silver-containing contact area is not required.

[0014] A further object of the present invention is preferably to provide a method for producing a metal-ceramic substrate having a silver-containing contact area, which is technically feasible with little effort.

[0015] A contribution to at least partially fulfilling at least one of the above tasks is made by a process for producing a metal-ceramic substrate having a silver-containing contact area, comprising the steps a) Providing a metal-ceramic substrate comprising a1) a ceramic body and a2) a metal layer bonded planarly to the ceramic body, wherein the ceramic of the ceramic body is selected from the group consisting of aluminum nitride ceramics, silicon nitride ceramics and aluminum oxide ceramics; b) Providing a silver ink comprising b1) a silver precursor, wherein the silver precursor is a silver carboxylate, and b2) an organic compound that is liquid at room temperature and ambient pressure, wherein the organic compound comprises a terpene, wherein the proportion of silver is in the range of 1 to 60 wt%, based on the weight of the silver ink; c) Applying the silver ink to an area of ​​the metal layer of the metal-ceramic substrate; and d) Decomposing the silver precursor contained in the silver ink to silver, obtaining a metal-ceramic substrate comprising a silver-containing contact area.

[0016] In the method according to the invention, a metal-ceramic substrate is provided.

[0017] The metal-ceramic substrate comprises a ceramic body.

[0018] The ceramic body is preferably a body made of ceramic. The body can have any geometry, but is preferably designed as a cuboid. The ceramic body has bounding surfaces; in the case of a cuboid, six bounding surfaces. The main bounding surface is preferably the one with the largest area that is bonded to the metal layer. The main bounding surface preferably lies in the main plane of extension or runs parallel to it. Accordingly, the main plane of extension of the ceramic body is preferably understood to be a plane that runs parallel to or encloses the main bounding surface of the ceramic body.

[0019] The ceramic of the ceramic body is preferably an insulating ceramic. The ceramic is selected from the group consisting of aluminum nitride ceramics, silicon nitride ceramics, and aluminum oxide ceramics (such as ZTA ("Zirconia Toughened Alumina") ceramics). According to a particularly preferred embodiment, the ceramic body consists of (1) at least one element selected from the group consisting of silicon and aluminum, (2) at least one element selected from the group consisting of oxygen and nitrogen, optionally (3) at least one element selected from the group consisting of (3a) rare earth metals, (3b) metals of group 2 of the periodic table, (3c) zirconium, (3d) copper, (3e) molybdenum, and (3f) silicon, and optionally (4) unavoidable impurities.According to yet another particularly preferred embodiment, the ceramic body is free of bismuth, gallium and zinc.

[0020] The ceramic body preferably has a thickness of 0.05 - 10 mm, more preferably in the range of 0.1 - 5 mm and particularly preferably in the range of 0.15 - 3 mm.

[0021] The metal-ceramic substrate comprises a metal layer that is bonded to the ceramic body over a flat surface.

[0022] The metal layer is preferably metallurgically bonded to the ceramic body. According to a preferred embodiment, the metal layer is bonded to the ceramic body via a DCB (Direct Copper Bonding) process. According to another preferred embodiment, the metal layer is bonded to the ceramic body via a brazing process. The brazing process can, for example, be an AMB (Active Metal Brazing) process, preferably using silver-free brazing alloys (the silver content is then preferably less than 1.0 wt. percent based on the solids content of the brazing alloy) or silver-containing brazing alloys (the silver content is then preferably at least 50 wt. percent based on the solids content of the brazing alloy). Consequently, the metal layer can also comprise a bonding layer that is in contact with the ceramic body.The bonding layer can be, for example, a solder layer (especially a hard solder layer) or a diffusion layer.

[0023] The metal layer is bonded to the ceramic body over a surface area. Accordingly, the metal layer is preferably bonded over a surface area to the main boundary surface of the ceramic. Preferably, the metal layer is not bonded to the entire main boundary surface of the ceramic. In particular, it may be provided that the main boundary surface of the ceramic body is larger than the area of ​​the metal layer bonded to the ceramic body. In these cases, the main boundary surface of the ceramic body protrudes. Furthermore, the metal layer is preferably structured. Structures are understood to be recesses in the metal layer to separate individual sections of the metal layer from one another and thus electrically insulate them. Such structures are typically created by etching techniques.

[0024] The metal of the metal layer is preferably selected from the group consisting of copper, aluminum, and molybdenum. According to a particularly preferred embodiment, the metal of the metal layer is selected from the group consisting of copper and molybdenum. According to a very particularly preferred embodiment, the metal of the metal layer is copper. According to a further very preferred embodiment, the metal layer consists of copper and unavoidable impurities.

[0025] The metal layer preferably has a thickness in the range of 0.01 - 10 mm, particularly preferably in the range of 0.03 - 5 mm and most preferably in the range of 0.05 - 3 mm.

[0026] In the method according to the invention, a silver ink is provided.

[0027] The silver ink contains at least one silver precursor.

[0028] Silver precursors preferably form a silver compound that can be decomposed while yielding silver.

[0029] According to a preferred embodiment, the decomposition of the silver precursor is carried out by exposing it to a decomposition temperature. According to a further preferred embodiment, the silver precursor has a decomposition temperature in the range of 50–500 °C, more preferably in the range of 80–500 °C, even more preferably in the range of 150–500 °C, particularly preferably in the range of 180–350 °C, and most preferably in the range of 150–300 °C, for example, in the range of 180–270 °C.

[0030] According to a further preferred embodiment, the decomposition of the silver precursor is achieved by exposing it to irradiation. According to yet another preferred embodiment, the silver precursor can be decomposed if at least 80%, and particularly preferably at least 90%, of the radiation has a wavelength in the range of 100 nm to 1 mm, in the range of 100 nm to 7 µm, in the range of 280 nm to 100 µm, in the range of 800 nm to 10 µm, in the range of 1 to 10 µm, in the range of 1 to 8 µm, or in the range of 1 to 5 µm, based on the total applied radiation.

[0031] According to the invention, the silver precursor has a cation and an anion.

[0032] The cation of the silver precursor is silver. The anion of the silver precursor is selected from the group consisting of carboxylates. The silver precursor is therefore a silver carboxylate.

[0033] The silver ink can contain one or more silver precursors. If the silver ink contains more than one silver precursor, it preferably comprises a combination of at least two silver carboxylates or a combination of at least one silver carboxylate and at least one silver carbamate.

[0034] A silver carboxylate is understood to be a carboxylic acid salt of silver consisting of at least one silver cation and at least one carboxylate anion. The carboxylate anion may be linear or branched, or may have cyclic structural elements. The carboxylate anion may be saturated or unsaturated. According to a preferred embodiment, the carboxylate is selected from the group consisting of monocarboxylates, dicarboxylates, cyclic carboxylates, and ketocarboxylates.

[0035] According to one embodiment, linear, saturated carboxylates are preferred, preferably carboxylates with 1 to 20 carbon atoms and particularly preferably carboxylates with 7 to 16 carbon atoms, the carbon atom of which always includes the carboxylate group. According to a preferred embodiment, the linear carboxylates are selected from the group consisting of acetate, propionate, butanoate, pentanoate, hexanoate, heptanoate, octanoate, nonanoate, decanoate, undecanoate, dodecanoate, tetradecanoate, hexadecanoate, and octadecanoate.

[0036] According to another embodiment, saturated isocarboxylates or saturated neocarboxylates with 1 to 20 carbon atoms, preferably with 7 to 16 carbon atoms, are used. In some embodiments, neocarboxylates with at least five carbon atoms are preferred. The neocarboxylates with at least five carbon atoms are preferably selected from the group consisting of neopentanoate, neohexanoate, neoheptanoate, neooctanoate, neononanoate, neodecanoate, and neododecanoate. Such neocarboxylates are produced, for example, by Shell or Exxon.

[0037] According to a preferred embodiment, the silver carboxylate is a linear, saturated silver carboxylate. According to a particularly preferred embodiment, the linear, saturated silver carboxylate is selected from the group consisting of silver acetate, silver propionate, silver butanoate, silver pentanate, silver hexanoate, silver heptanoate, silver octanoate, silver nonanoate, silver decanoate, silver undecanoate, silver dodecanoate, silver tetradecanoate, silver hexadecanoate, and silver octadecanoate.

[0038] According to a further preferred embodiment, the silver carboxylate is selected from the group consisting of saturated silver isocarboxylates and saturated silver neocarboxylates. According to a particularly preferred embodiment, the saturated silver neocarboxylate is selected from the group consisting of silver neopentanoate, silver neohexanoate, silver neoheptanoate, silver neooctanoate, silver neononanoate, silver neodecanoate, and silver neododecanoate.

[0039] According to a particularly preferred embodiment, the silver carboxylate is a silver neodecanoate.

[0040] According to a preferred embodiment, the silver precursor is a solid at room temperature. If the silver ink according to the invention is a liquid or a mixture with solid and liquid components, at least part of the silver precursor may have dissolved completely.

[0041] The silver content ranges from 1 to 60 percent by weight of the silver ink.

[0042] According to a preferred embodiment, the silver content is in the range of 1–50% by weight, for example, in the range of 10–30% or 30–50% by weight, based on the weight of the silver ink. Preferably, the silver content, based on the weight of the silver ink, is determined by thermogravimetric analysis (TGA).

[0043] According to a further preferred embodiment, the proportion of at least one silver carboxylate is in the range of 10–80 percent by weight, more preferably in the range of 20–80 percent by weight, particularly preferably in the range of 20–60 percent by weight and most preferably in the range of 20–45 percent by weight, based on the weight of the silver ink.

[0044] The silver ink contains at least one organic compound that is liquid at room temperature and ambient pressure, wherein the organic compound comprises a terpene.

[0045] The room temperature is 20 °C (+293 K). The term ambient pressure refers to an absolute pressure of 1013 hPa. The standard relative humidity in this context is 50% RH. An organic compound is preferably considered liquid if it has a viscosity in the range of 1 to 300 mPas, determined according to DIN 53019-1:2008 at 20 °C. The viscosity is preferably measured using a Brookfield DV3 rheometer, spindle no. 21, at 100 rpm and 20 °C.

[0046] The organic compound comprises at least one terpene. According to a particularly preferred embodiment, the organic compound comprises at least one terpene containing 5 to 20 carbon atoms, and most preferably 5 to 12 carbon atoms.

[0047] A terpene is preferably a naturally occurring unsaturated hydrocarbon that can be isolated from natural substances and whose structure can be traced back to one or more isoprene units. Some terpenes can now also be produced industrially and synthetically. The polymers of cis- and trans-polyisoprene are generally not considered terpenes. The terpene is preferably an acyclic terpene or a cyclic terpene. Among the cyclic terpenes, monocyclic terpenes are preferred.

[0048] According to a preferred embodiment, the terpene is selected from the group consisting of orange terpenes, limonene and pinene.

[0049] Orange terpene is a liquid terpene mixture with a boiling point in the range of 170 to 180°C (p = 1 bar), typically obtained by cold pressing orange peels. Orange terpene contains more than 80 wt%, preferably more than 85 wt%, and particularly preferably more than 90 wt% of (+) / (-)-limonene. Other common components of orange terpenes are aldehydes such as octanal, decanal, sinensal, as well as octyl acetate and neryl acetate. One supplier of orange terpene (CAS No. 68647-72-3) is Carl Roth GmbH, 76231 Karlsruhe, Germany.

[0050] The limonene in this case preferably contains (r)-(+)-limonene, (s)-(-)-limonene or a mixture of both in any ratio, for example a racemic mixture.

[0051] In the present case, pinene is preferably a monoterpene hydrocarbon with the molecular formula C10H16. A particularly preferred pinene is β-pinene (CAS No. 19902-08-0).

[0052] According to a preferred embodiment, the silver ink contains at least one further component. This at least one further component is preferably selected from the group consisting of adhesion promoters, viscosity aids, organic solvents and other additives.

[0053] According to another preferred embodiment, the silver ink contains at least one adhesion promoter.

[0054] The at least one adhesion promoter is preferably selected from the group of rhodium alkyl compounds. Rhodium alkyl compounds are rhodium compounds containing at least one alkyl group. If a rhodium alkyl compound contains more than one alkyl group, for example, two or three alkyl groups, these alkyl groups can be the same or different. The at least one alkyl group preferably has a length of 2 to 20 carbon atoms and particularly preferably a length of 12, 14, 16, or 18 carbon atoms. A particularly preferred adhesion promoter is rhodium nonanoate.

[0055] According to a preferred embodiment, the proportion of the at least one adhesion promoter is in the range of 0.1 - 5 percent by weight, based on the weight of the silver ink.

[0056] According to another preferred embodiment, the silver ink contains at least one viscosity aid.

[0057] Rosin resins or their derivatives are particularly suitable as viscosity aids for ink composition. A particularly preferred commercial product is balsam resin, available from H. Reynaud & Fils GmbH, Hamburg.

[0058] According to a preferred embodiment, the proportion of the at least one viscosity aid is in the range of 5 - 30 percent by weight and particularly preferably in the range of 10 - 20 percent by weight, based on the weight of the silver ink.

[0059] According to another preferred embodiment, the silver ink contains at least one organic solvent.

[0060] The at least one organic solvent is preferably selected from the group of glycol ethers. Glycol ethers are organic substances containing at least one ethylene glycol unit or one diethylene glycol unit. Propylene glycol ethers or dipropylene glycol ethers are particularly preferred as glycol ethers. Commercially available examples are Dowanol PnP (propylene glycol n-propyl ether), Dowanol PnB (propylene glycol n-butyl ether), Dowanol DPnB (dipropylene glycol n-butyl ether), and Dowanol DPnP (dipropylene glycol n-propyl ether).

[0061] According to a preferred embodiment, the proportion of the at least one organic solvent is in the range of 0.1 to 80 percent by weight, particularly preferably in the range of 20 to 80 percent by weight and most preferably in the range of 20 to 50 percent by weight, based on the weight of the silver ink.

[0062] According to another preferred embodiment, the silver ink contains at least one additive.

[0063] The at least one additive can, for example, be a carboxylic acid. In principle, the carboxylic acid can be any carboxylic acid known to a person skilled in the art and deemed suitable. Preferably, the carboxylic acid is a tertiary carboxylic acid with at least five carbon atoms. According to a particularly preferred embodiment, the tertiary carboxylic acid is selected from the group consisting of neopentanoic acid, neohexanoic acid, neoheptanoic acid, neooctanoic acid, neononanoic acid, neodecanoic acid, and neododecanoic acid. According to a very particularly preferred embodiment, the additive comprises neodecanoic acid. Neocarboxylic acids are produced, for example, by Shell or Exxon. By adding a carboxylic acid or a combination of several carboxylic acids, the shelf life of the ink composition can be extended.

[0064] Other additives include all chemical substances and mixtures known to those skilled in the art that appear suitable for the intended purpose. Silicone-containing additives, such as polyether-modified polydimethylsiloxanes, are particularly preferred. Such additives are available, for example, under the brand name BYK from BYK Additives & Instruments GmbH, 46483 Wesel, Germany.

[0065] According to a preferred embodiment, the proportion of the at least one additive is in the range of 0.05 - 3 percent by weight and particularly preferably in the range of 0.05 - 1 percent by weight, based on the weight of the silver ink.

[0066] The components of the silver ink can be mixed in any way known to a person skilled in the art and deemed suitable by them. Mixing can be carried out at slightly elevated temperatures to facilitate the process. Generally, the temperature during mixing does not exceed 40°C. The silver ink can be stored at room temperature or in a refrigerator.

[0067] According to a preferred embodiment, the proportion of the sum of the at least one silver precursor and the at least one organic compound, which is liquid at room temperature and ambient pressure, is at least 80% by weight, more preferably at least 90% by weight, particularly preferably at least 95% by weight and most preferably at least 99% by weight, based on the weight of the silver ink.

[0068] According to a further preferred embodiment, the proportion of the sum of the at least one silver precursor and terpenes is at least 80% by weight, more preferably at least 90% by weight, particularly preferably at least 95% by weight and most preferably at least 99% by weight, based on the weight of the silver ink.

[0069] According to a further preferred embodiment, the proportion of the sum of at least one silver carboxylate and terpenes is at least 80% by weight, more preferably at least 90% by weight, particularly preferably at least 95% by weight and most preferably at least 99% by weight, based on the weight of the silver ink.

[0070] According to a further preferred embodiment, the proportion of the sum of at least one silver carboxylate and terpenes is in the range of 10 - 95 percent by weight and particularly preferably in the range of 10 - 80 percent by weight, based on the weight of the silver ink.

[0071] According to a further preferred embodiment, the proportion of the sum of at least one silver carboxylate and terpenes is in the range of 35–95 percent by weight, more preferably in the range of 35–80 percent by weight, particularly preferably in the range of 40–60 percent by weight and most preferably in the range of 45–55 percent by weight, based on the weight of the silver ink.

[0072] According to a particularly preferred embodiment, the ratio of (i) at least one silver carboxylate to (ii) at least one terpene in the silver ink is in the range of 4:1 to 1:5, more preferably in the range of 2:1 to 1:2, particularly preferably in the range of 1:1 to 1:2, and most preferably in the range of 1:1.5 to 1:2. The corresponding values ​​can preferably be determined by HPLC measurement, wherein a characteristic signal is selected for each component and compared.

[0073] According to a further preferred embodiment, the weight ratio of (i) silver to (ii) the sum of the elements gold, rhodium, vanadium, palladium, platinum, osmium, copper, tungsten, bismuth, silicon, zirconium dioxide, tin, copper and aluminium in the silver ink is at least 100 : 1 and particularly preferably at least 150 : 1, based on the weight fractions in the silver ink.

[0074] According to a further preferred embodiment, the weight ratio of (i) silver to (ii) rhodium in the silver ink is at least 100 : 1 and particularly preferably at least 150 : 1, based on the weight fractions in the silver ink.

[0075] According to a further preferred embodiment, the total proportion of the elements selected from the group consisting of gold, rhodium, vanadium, palladium, platinum, osmium, tungsten, bismuth, silicon, zirconium dioxide, tin, copper and aluminium is less than 10% by weight, more preferably less than 8% by weight, even more preferably less than 6% by weight, particularly preferably less than 5% by weight and most particularly preferably less than 2% by weight, for example less than 1% by weight, based on the weight of the silver ink.

[0076] According to a further preferred embodiment, the total proportion of the elements selected from the group consisting of rhodium, vanadium and osmium is less than 1 percent by weight, more preferably less than 0.5 percent by weight and particularly preferably less than 0.2 percent by weight, based on the weight of the silver ink.

[0077] According to a further preferred embodiment, the proportion of metal particles in the silver ink is less than 1 percent by weight, particularly preferably less than 0.5 percent by weight, and most preferably less than 0.2 percent by weight, based on the weight of the silver ink. Thus, the silver ink according to the invention contains practically no metal particles. In this context, metal particles are always solids.

[0078] According to a further preferred embodiment, the silver ink has a viscosity in the range of 0.1–100 mPas, and particularly preferably in the range of 20–70 mPas, determined at a temperature of 20 °C and an ambient pressure of 1013 hPa. The viscosity is preferably measured with a Brookfield DV3 rheometer, spindle no. 21, at 100 rpm and 20 °C.

[0079] In the method according to the invention, the silver ink is applied to an area of ​​the metal layer of the metal-ceramic substrate.

[0080] For this purpose, the silver ink is preferably applied to at least the area of ​​the metal layer of the metal-ceramic substrate on which the silver-containing contact area is to be created.

[0081] The silver ink can be applied to the metal layer of the metal-ceramic substrate using standard techniques. These include, for example, application by printing, spraying, brushing, dispensing, and writing and transferring the silver ink from a stencil onto the metal-ceramic substrate, such as in transfer printing. Preferably, the silver ink is applied to at least the metal layer of the metal-ceramic substrate by printing, particularly preferably by screen printing or inkjet printing. According to a particularly preferred embodiment, the silver ink is applied to a portion of the metal layer of the metal-ceramic substrate by inkjet printing. An inkjet printer is preferably used for this purpose. The Fuji Dimatix 2850 inkjet printer is an example.

[0082] The wet film thickness of the silver ink applied to a region of the metal layer is not further limited. According to a preferred embodiment, the mean wet film thickness of the silver ink applied to a region of the metal layer of the metal-ceramic substrate is in the range of 0.5–30 µm and particularly preferably in the range of 1–20 µm.

[0083] In the process according to the invention, the silver precursor contained in the silver ink is decomposed to silver, yielding a metal-ceramic substrate having a silver-containing contact area.

[0084] According to a preferred embodiment, the decomposition of the silver precursor contained in the silver ink is carried out by at least one measure selected from the group consisting of thermal treatment and irradiation. According to a further preferred embodiment, the decomposition of the silver precursor contained in the silver ink is carried out by a combination of measures, particularly preferably by a combination of thermal treatment and irradiation, and most preferably by a sequential combination of irradiation and subsequent thermal treatment.

[0085] According to a particularly preferred embodiment, the decomposition of the silver precursor contained in the silver ink is carried out by thermal treatment. For this purpose, the silver ink is preferably heated. The heating is preferably carried out to a temperature and for a time that is necessary for the silver precursor to decompose in order to obtain a silver-containing contact area on the metal layer of the metal-ceramic substrate.

[0086] According to a further preferred embodiment, the heating is carried out to a temperature of at least 50°C, more preferably to a temperature of at least 100°C, even more preferably to a temperature of at least 150°C, particularly preferably to a temperature of at least 200°C and most preferably to a temperature of at least 250°C.

[0087] According to a further preferred embodiment, the heating is carried out to a temperature of no more than 800°C, more preferably to a temperature of no more than 700°C, even more preferably to a temperature of no more than 600°C, particularly preferably to a temperature of no more than 500°C and most particularly preferably to a temperature of no more than 400°C.

[0088] According to a further preferred embodiment, heating is carried out to a temperature in the range of 50 - 800 °C, more preferably to a temperature in the range of 100 - 700 °C, even more preferably to a temperature in the range of 150 - 600 °C, particularly preferably to a temperature in the range of 200 - 500 °C and most particularly preferably to a temperature in the range of 250 - 400 °C.

[0089] According to a further preferred embodiment, the heating takes place for a period of at least 10 minutes, particularly preferably for a period of at least 12 minutes and most preferably for a period of at least 15 minutes.

[0090] According to a further preferred embodiment, the heating takes place for a period of no more than 300 minutes, particularly preferably for a period of no more than 120 minutes and most preferably for a period of no more than 60 minutes.

[0091] According to a further preferred embodiment, the heating takes place for a period of time in the range of 10 - 300 minutes, particularly preferably for a period of time of 12 - 120 minutes and most preferably for a period of time of 15 - 60 minutes.

[0092] According to yet another preferred embodiment, the heating takes place in an air atmosphere.

[0093] According to a particularly preferred embodiment, the decomposition of the silver precursor contained in the silver ink is carried out by irradiation.

[0094] According to a further preferred embodiment, the decomposition of the silver precursor contained in the silver ink is carried out by irradiation, wherein at least 80%, particularly preferably at least 90% and most preferably 100% of the radiation has a wavelength in the range of 100 nm - 1 mm, more preferably a wavelength in the range of 100 nm - 7 µm, even more preferably a wavelength in the range of 280 nm - 100 µm, particularly preferably a wavelength in the range of 800 nm - 10 µm and most preferably a wavelength in the range of 1 to 10 µm, for example a wavelength in the range of 1 - 8 µm or a wavelength in the range of 1 - 5 µm, based on the total radiation applied during the treatment.

[0095] According to a particularly preferred embodiment, the decomposition of the silver precursor contained in the silver ink is carried out by infrared irradiation, preferably with a peak wavelength in the range of 1500 nm - 4000 nm and particularly preferably with a peak wavelength in the range of 2000 - 3000 nm or a peak wavelength in the range of 2800 - 3300 nm.

[0096] The wavelength distribution of an irradiation applied to decompose the silver precursor contained in the silver ink, the percentage of the irradiation (area within two cutoff wavelengths relative to the total area under the spectral curve and y = 0), and the peaks in the distribution are preferably determined using a Thorlabs Optical Spectrum Analyzer OSA207C: 1.0 - 12.0 µm in combination with a hollow-core fiber, available from Opto-Knowledge Systems, Inc. in Torrance (CA), USA, by bringing one end of this fiber close to the surface of the emitter. The wavelength distribution is determined according to the FT-IR measurement principle.

[0097] According to a further preferred embodiment, the decomposition of the silver precursor contained in the silver ink is carried out by irradiation for a period of less than 10 minutes, more preferably for a period of less than 5 minutes, even more preferably for a period of less than 2 minutes, particularly preferably for a period of less than 60 seconds and most preferably for a period of less than 30 seconds, for example for a period of less than 15 seconds.

[0098] According to yet another preferred embodiment, the decomposition of the silver precursor contained in the silver ink is carried out by irradiation at a total radiant flux density in the range of 100 to 1,000,000 W / m². To determine the total radiant flux density, the total power consumption of the emitters generating the radiation is measured and the obtained value is divided by the illuminated area.

[0099] By decomposing the silver precursor contained in the silver ink into silver, a silver-containing contact area is created on the metal-ceramic substrate.

[0100] Accordingly, the metal-ceramic substrate has a silver-containing contact area arranged on the metal layer. This contact area preferably serves to facilitate the attachment of a component, for example, a chip, to the metal layer. Chips are preferably connected to the metal layer by sintering, soldering, or bonding. Since attaching chips to the metal of the metal layer of a metal-ceramic substrate is not always straightforward, the metal layer is preferably provided with a contact area. Preferably, a contact area is provided on the metal layer of the metal-ceramic substrate at all positions where components, such as chips, are to be subsequently placed on the metal-ceramic substrate.

[0101] Surprisingly, it was found that metal-ceramic substrates obtained according to the inventive method exhibit, on the one hand, high reliability under temperature cycling (in terms of a stable bond between the metal layer and the ceramic body) and, on the other hand, can be bonded to components, in particular electronic components and contacting elements (such as bonding wires), with high adhesive strength. The bonding of electronic components preferably occurs via the contact area, while the bonding of the contacting elements preferably occurs directly on the metal layer.

[0102] Without being bound to an explanation, this could be due to the fact that, in contrast to the prior art methods, the inventive method does not damage either the surface of the metal layer of the metal-ceramic substrate or an area of ​​the metal layer of the metal-ceramic substrate near the ceramic body.

[0103] The invention therefore further provides a metal-ceramic substrate obtainable according to the inventive method. The creation of a contact area comprising silver facilitates easier bonding of the chip to the metal-ceramic substrate using common methods such as sintering or soldering. The resulting metal-ceramic substrate is characterized by high reliability under temperature cycling (in the sense of a stable bond between the metal layer and the ceramic body). Furthermore, the resulting metal-ceramic substrate enables bonding with components to be connected, in particular electronic components and contacting elements (such as bond wires), with high adhesive strength. Examples of implementation

[0104] The present invention is described in more detail below by means of exemplary embodiments, which, however, should not be understood as limiting. Example:

[0105] For the example, a metal-ceramic substrate was used in which a ceramic body made of an aluminum oxide ceramic with dimensions of 177.8 x 139 x 0.32 mm was bonded on both sides to a copper layer with dimensions of 170 x 132 x 0.3 mm using a DCB (Direct Copper Bonding) process.

[0106] Before the application of the silver-containing contact areas, both copper layers were structured in the same way so that isolated copper areas measuring 15 x 13 mm were equidistant from each other on the aluminum oxide ceramic.

[0107] A silver ink with the following composition was applied to the isolated copper areas: 50 wt% orange terpene, 47 wt% silver neodecanoate (Ag Versatic; with a silver content of 38 wt%), and 3 wt% carboxylic acid (Versatic carboxylic acid, HEXION, Rotterdam, Netherlands). The silver ink was applied to the center of each copper area, measuring 5 x 5 mm, using inkjet printing (Fuji Dimatix Samba cartridge). The wet film thickness was 4.4 µm. Within 30 seconds of printing, the ink was irradiated with UV LED lamps at a wavelength of 385 nm for 90 s. The UV radiation intensity was adjusted to achieve a temperature of 180 °C on the metal-ceramic substrate. The pretreated metal-ceramic substrate was then heated in an oven under a nitrogen atmosphere.For this purpose, the temperature was increased from room temperature to 250 °C at a heating rate of 10 °C / min and, after one hour at 250 °C, reduced to 25 °C at a cooling rate of 10 °C / min.

[0108] To test the suitability of the silver-containing contact areas according to the invention for chip interconnection, a silver sintering paste (mAgic® PE338 from Heraeus) was applied to the center of each of the contact areas created with the silver ink using stencil printing over an area of ​​4 x 4 mm. The wet film thickness was 50 µm. The printed sintering paste was then dried in normal air atmosphere for 10 minutes at 120 °C, and subsequently each area was fully contacted with a silicon chip with a silver contact area (4 x 4 mm). The subsequent pressure sintering of this arrangement was carried out under a nitrogen atmosphere (<100 ppm oxygen) in a hot press at 230 °C and 12 MPa for 5 minutes. Comparison example:

[0109] For the comparative example, a metal-ceramic substrate was used in which a ceramic body made of an aluminum oxide ceramic with dimensions of 177.8 x 139 x 0.32 mm was bonded on both sides to a copper layer with dimensions of 170 x 132 x 0.3 mm using a DCB (Direct Copper Bonding) process.

[0110] Before the application of the silver-containing contact areas, both copper layers were structured in the same way so that isolated copper areas measuring 15 x 13 mm were equidistant from each other on the aluminum oxide ceramic.

[0111] Subsequently, silver-containing contact areas were produced via electrochemical deposition. For this purpose, a photosensitive film was first applied to all copper areas of a copper layer using a hot roll laminator. The photosensitive film was exposed to light at 30 mJ / cm² in the areas to be masked in order to harden the polymer contained in the photosensitive film and obtain the mask. The unexposed areas of the photosensitive film were then removed using a wet chemical process with a sodium carbonate solution (concentration = 10 g / l). After a rinsing step for cleaning, silver-containing contact areas were deposited onto the unmasked areas of the copper layers of the metal-ceramic substrate. These contact areas corresponded in number, position, and dimensions to the silver-containing contact areas of the example according to the invention.For this purpose, the masked metal-ceramic substrate was first pretreated with a solution containing hydrogen peroxide and sulfuric acid and then contacted with a silver nitrate solution (silver content = 1.0 g / l). After deposition of the silver-containing contact areas, the metal-ceramic substrate was thoroughly rinsed with water to remove any residues. Subsequently, the mask was removed in a stripping unit using a 2.5% sodium hydroxide solution, and the metal-ceramic substrate was thoroughly rinsed with water.

[0112] In the comparative example, the same silver sintering paste was printed onto each of the silver-containing contact areas in the same positions and dimensions and, as described in the example according to the invention, dried, fitted with chips and then sintered.

[0113] Both metal-ceramic substrates (both the example and the comparison example) were cut into individual parts with dimensions (20.5 x 17.0 mm) using a laser and then used for further investigations. Evaluation: Temperature cycling resistance test:

[0114] To prepare for the thermal shock resistance test, the individual metal-ceramic substrates were first checked for defects using ultrasonic microscopy (PVA Tepla SAM300). Only metal-ceramic substrates were used for the test that showed no delamination between the ceramic body and the metal layer, or any other deformations that could lead to delamination of the metal layer from the ceramic body (e.g., cracks). To test the thermal shock resistance, the metal-ceramic substrates were repeatedly exposed to a cold liquid (temperature -65°C, Galden Do2TS) and a hot liquid (temperature +150°C, Galden Do2TS) in a cycling chamber (ESPEC TSB-21 51), each for a period of five minutes. The metal-ceramic substrates were re-inspected for delamination and other deformations using ultrasonic microscopy (PVA Tepla SAM300) every 1000 cycles. The test was terminated after 3000 cycles.The metal-ceramic substrates were then examined again for delamination and other deformations using ultrasonic microscopy (PVA Tepla SAM300). The condition of the respective metal-ceramic substrates after the thermal cycling test was compared with the condition of the metal-ceramic substrates before the thermal cycling test with regard to delamination and other deformations. Delaminations and other deformations (e.g., cracks) were visible as white discolorations in the ultrasonic image. The results are shown in Table 1. Shear strength test:

[0115] Shear strength tests were performed to determine the adhesive strength of the chips on the respective silver-containing contact areas of the metal-ceramic substrates. For this purpose, the shear force required to shear the chip off the metal-ceramic substrate at a shearing speed of 0.3 mm / s at 20 °C was measured. The force was recorded using a load cell (DAGE 2000, DAGE, Germany). The results are also shown in Table 1. Table 1: Result of the temperature cycling resistance test Result of the shear strength test Example Positive; no delaminations visible 45 - 55 N / mm²< Comparison example Negative; pronounced delamination is particularly noticeable at the copper edges. 42 - 49 N / mm²

[0116] The results show that the metal-ceramic substrate with silver-containing contact areas obtained by the inventive method is significantly superior to the metal-ceramic substrate of the comparison example with regard to temperature cycling resistance and adhesion strength.

Claims

1. A method for producing a metal-ceramic substrate comprising a silver-containing contact region, the method comprising the steps of a) providing a metal-ceramic substrate comprising a1) a ceramic body and a2) a metal layer connected in a planar manner to the ceramic body, wherein the ceramic of the ceramic body is selected from the group consisting of aluminum nitride ceramics, silicon nitride ceramics, and aluminum oxide ceramics, b) providing a silver ink containing b1) a silver precursor, wherein the silver precursor is a silver carboxylate, and b2) an organic compound which is liquid at room temperature and ambient pressure, wherein the organic compound comprises a terpene, wherein the proportion of silver is in the range of 1 - 60 percent by weight, based on the weight of the silver ink, c) applying the silver ink to a region of the metal layer of the metal-ceramic substrate and d) decomposing the silver precursor contained in the silver ink to form silver to obtain a metal-ceramic substrate comprising a silver-containing contact region.

2. The method according to claim 1, characterized in that the metal of the metal layer is copper.

3. The method according to either claim 1 or claim 2, characterized in that the silver carboxylate is a silver neodecanoate.

4. The method according to any of the preceding claims, characterized in that the terpene is selected from the group consisting of orange terpene, limonene and pinene.

5. The method according to any of the preceding claims, characterized in that the silver ink contains a carboxylic acid.

6. The method according to any of the preceding claims, characterized in that the silver precursor contained in the silver ink is decomposed by at least one measure selected from the group consisting of thermal treatment and irradiation.