DE102010025311A1 - Formation of metallic layer on ceramic substrate or ceramic layer, involves melting interfacial layer formed from eutectic mixture formed using oxide of metallic material and alumina, on ceramic substrate or ceramic layer - Google Patents

Abstract

A metallic film or metallic layer is formed by melting an interfacial layer formed from eutectic mixture, on ceramic substrate or ceramic layer. The eutectic mixture is formed using oxide of metallic material and alumina as ceramic filler. An independent claim is included for composite.

Description

Technical application

The present invention relates to a method for applying a metallic layer to a substrate or a layer of a ceramic material, wherein a foil or plate consisting of a metallic material, by melting an intermediate layer of a eutectic mixture with the substrate or layer of the ceramic material is connected.

Ceramic substrates with metallization are used, for example, as a circuit carrier power electronic modules in which they ensure the thermal and mechanical connection and the electrical insulation. The lifetime of the substrates provided with the metallization is limited by the different thermal expansion coefficients of substrate and metallization. These induce mechanical stresses in the substrate during thermal cycles and thus lead to fatigue fractures.

State of the art

• – Oxidieren einer Kupferfolie derart, dass sich eine gleichmäßige Kupferoxidschicht bildet;
• – Auflegen der Kupferfolie auf die Keramikschicht;
• – Erhitzen des Verbundes auf eine Prozesstemperatur zwischen etwa 1025 und 1083°C, z. B. auf ca. 1071°C; und
• – Abkühlen auf Raumtemperatur.

The application of a metallization on a ceramic substrate can be done with a technique such as in the DE 23 19 854 A is described. Here, a metal part, for example. A copper plate or copper foil, on the surface sides with a coating of a chemical compound of the metal and a reactive gas, in particular oxygen, provided. This coating forms a eutectic with a thin layer of the adjacent metal having a melting temperature below the melting temperature of the metal. The metal part is then placed on the ceramic substrate and heated together with the ceramic to a temperature above the melting point of the eutectic and below the melting temperature of the metal. As a result, essentially only the eutectic intermediate layer is melted. After cooling, the metal part and the ceramic substrate are then joined together. When copper or a copper alloy is used as metal, this process is also referred to as DCB bonding or DCB (Direct Copper Bonding) process, but it can also be performed with other metals (eg, aluminum). The DCB method comprises, for example, the following method steps:

• - Oxidizing a copper foil such that forms a uniform copper oxide layer;
• - placing the copper foil on the ceramic layer;
• - Heating the composite to a process temperature between about 1025 and 1083 ° C, z. B. to about 1071 ° C; and
• - Cool to room temperature.

The resulting composite material, d. H. the metal-ceramic substrate can then be further processed in the desired manner.

The prerequisite for the formation of the required in the process intermediate layer of a eutectic mixture is in the DE 23 19 854 A created by a thermal oxidation of the metal part. Another technique for forming the intermediate layer between a copper foil and a ceramic substrate is shown in FIG Y. Yoshino et al., "Interface Structure and Bond Strength of Copper-Bonded Alumina Substrate", Journal of the American Ceramic Society, Vol. 9, 1991, pages 2184-2188 , described. In this technique, as a solid source for the formation of the intermediate layer, a CuO paste is first applied to the ceramic substrate. Subsequently, the copper foil is placed and connected by melting the intermediate layer with the ceramic substrate.

Due to the different coefficients of thermal expansion of metallization or metal layer and substrate, fatigue fractures occur in the substrate during thermal cycles due to the thermal stresses. To reduce the voltages it is, for example, from the DE 43 18 241 A1 It is known to introduce structures into the edge region of the metal layer, by which the occurrence of cracks is delayed.

Another possibility is the use of particularly crack resistant or resistant ceramic materials such as Si ₃ N ₄ or zirconium doped alumina. Another known technique for increasing the reliability of the metal-ceramic composite is the use of plastically flowing metallizations, such as aluminum, in the possible cracking zone.

The object of the present invention is to specify a method for applying a metallic layer to a substrate or a layer of a ceramic material, which has a higher reliability or service life of the composite material thus obtained with respect to thermal stresses.

Presentation of the invention

The object is achieved by the method according to claim 1. Advantageous embodiments of the method are the subject of the dependent claims or can be found in the following description and the embodiment.

The proposed method uses the known bonding technique in which a foil or plate made of a metallic material is melted by melting an intermediate layer of a eutectic mixture with the substrate or the layer of the ceramic material. The method is characterized in that a mixture of at least the metallic material, an oxide of the metallic material and Al ₂ O ₃ as a ceramic filler between the film or plate and the substrate or the layer of the ceramic material is brought to form the intermediate layer ,

By virtue of the composition of this mixture, which forms a solid source for the formation of the intermediate layer of the eutectic mixture, a coefficient of thermal expansion is applied to the adjacent layers, i. H. the metallic and the ceramic layer, adapted intermediate layer achieved. This leads to lower stress loads under thermal loads of the composite material and thus to increased reliability in thermal cycles. This applies in particular to the use of the method for the production of metal-ceramic substrates or ceramic substrates with metallizations, which are used as printed circuit boards or supports for electrical and electronic circuits or modules, especially for circuits or modules with high performance. The metallization can be structured to form tracks or connections for the electrical or electronic circuits. The metallization may also serve, for example as a back-side metallization of the substrate, the dissipation of the heat.

The mixture is preferably obtained by mixing the metallic material, the oxide of the metallic material and the ceramic filler, each in powder form. By means of the proportion of the ceramic filler, the coefficient of thermal expansion can be adjusted in the desired manner, ie as close as possible to the mean value of the thermal expansion coefficients of the metallic foil or plate and of the substrate or layer of the ceramic material. In a further embodiment, one or more further ceramic materials, preferably as ceramic powder, are added to the mixture in order to adjust the resulting coefficient of thermal expansion in the desired manner. In this case, the Al ₂ O _{3 may,} for example, also be present as a coating of another ceramic powder. The mixture can be applied in powder form, but preferably as a paste, to the film or plate and / or to the substrate or layer. For this purpose, conventional application methods such as screen printing o. Ä. Or spray technology can be used.

The composite material obtained by the method has accordingly an intermediate layer of a eutectic mixture between the metallic layer and the ceramic layer, which is formed by melting from the applied mixture and an edge layer of the metallic plate or foil. The layer of the metallic material can then be patterned in a suitable manner, for example by means of photolithography in conjunction with an etching technique, in order to form conductor tracks, for example. With such a structure, the mixture, for example by screen printing, can already be applied to the ceramic substrate or the ceramic layer in order subsequently to shorten the time for structuring the metallic layer down to the ceramic material. This requires fewer process steps and thus saves costs.

Of course, metallic layers can also be applied on both sides of the substrate in the proposed method, so that, for example, a front side for the electrical contacting and connection of electrical or electronic circuits and the back can be used for improved heat dissipation.

Due to the adapted thermal expansion coefficient of the intermediate layer, which serves as an intermediary between ceramic and metallization, the resistance to thermal cycling is significantly increased. This allows for the same thickness ratio of the metallic layer and the ceramic layer more reliable substrates than in the previously known DCB process. Alternatively, while maintaining the reliability with respect to thermal cycles, the metallization thickness and thus heat spread and current carrying capacity can be further increased as a performance parameter of the substrate.

By using mixtures of the metal, metal oxide and ceramic admixtures, the flow properties and the thickness of the intermediate layer can be controlled compared to pure metal oxide as an oxygen carrier. As a result, thicker interlayers and a larger amount of locally available oxygen introduced into the bonding process compared to other oxidation methods are achieved. This allows the application of thicker metal layers, for example thicker copper layers in the DCB process.

Compared to the known thermal and chemical oxidation processes, which provide the metallization on all sides with an oxide layer, the local provision of oxygen in the form of a powder or a paste has the advantage that the metallization surfaces later provided as solder and wire bonding surfaces in a power module do not process, and thus not be influenced in their surface properties. This minimizes the Effort for the bonding process downstream cleaning and conditioning processes of the substrate surface, such as, for example, grinding or brushing.

Brief description of the drawings

The proposed method will be explained in more detail using exemplary embodiments in conjunction with the drawings. Hereby show:

1 a first example of an implementation of the proposed method; and

2 a second example of an implementation of the proposed method.

Ways to carry out the invention

Two examples of depositing a copper metallization onto an aluminum nitride (AlN) or alumina (Al ₂ O ₃ ) ceramic substrate are described below. Here, the known DCB process is used. Of course, however, the proposed method can also be used with other metals or metallic alloys and other ceramics. Prerequisite here is that the applied mixture allows the melting of a formation of a eutectic mixture having a lower melting point than the metal or the metal alloy. Furthermore, instead of the ceramic substrate, a substrate made of another material with a ceramic surface layer can be used.

In the present example, first a mixture 2 made of copper oxide, copper and the ceramic powder by mixing. The copper oxide may be CuO or Cu ₂ O. The mixed ceramic powder is an Al ₂ O ₃ powder - here in a ceramic substrate made of this material. Exemplary compositions of the mixture are in the range of up to 20% by weight of the metal, 10-100% by weight of the metal oxide, and up to 90% by weight of the ceramic powder. Alternatively, the admixed ceramic powder can also be, for example, another ceramic powder provided with Al ₂ O ₃ coating, for example AlN or Si ₃ N ₄ . This admixture of an additional ceramic material serves to adjust the resulting coefficient of thermal expansion. Depending on the method chosen, a readily volatile binder can also be mixed in order to increase the processability.

Through this mixture 2 For example, the amount of oxygen required for the DCB bonding process is provided locally between the metallic layer and the ceramic layer. The mixture 2 This is done in one step on the ceramic substrate 1 applied as in the 1 is shown schematically. This can be done, for example, by screen printing.

Following the application of the mixture 2 , in the present example of a paste, a copper sheet 3 mechanically applied. The layer stack is then heated in accordance with the DCB process to a temperature which is above the melting temperature of the intermediate layer and below the melting temperature of the copper sheet. The process temperature in this example is approximately between 1025 and 1083 ° C., for example 1071 ° C. This forms the intermediate layer 4 from the eutectic mixture. After cooling, the copper sheet 3 and the ceramic substrate 1 over the intermediate layer 4 connected with each other. Subsequently, a structuring of the metallization by conventional etching processes take place, as in the last step of 1 is indicated.

In the example of 1 became the mixture 2 for the intermediate layer over the entire surface of the surface of the ceramic substrate 1 applied. This can already be done in a structured way, as in 2 is indicated. This structured application of the Cu / CuO / ceramic paste can be achieved, for example, by means of screen printing. The further processing then takes place as already described in connection with 1 explained. After connecting the copper sheet 3 through the DCB process with the ceramic substrate 1 then can the copper sheet 3 be structured in the form as the structured intermediate layer 4 , This structuring of the metallization takes place again via known conventional etching processes.

LIST OF REFERENCE NUMBERS

1

ceramic substrate

2

Mixture for forming the intermediate layer

3

copper sheet

4

interlayer

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 2319854 A [0003, 0005]
• DE 4318241 A1 [0006]

Cited non-patent literature

• Y. Yoshino et al., "Interface Structure and Bond Strength of Copper-Bonded Alumina Substrate", Journal of the American Ceramic Society, Vol. 9, 1991, pages 2184-2188 [0005]

Claims (9)

Method for applying a metallic layer to a substrate or a layer of a ceramic material, in which a foil or plate, which consists of a metallic material, by melting an intermediate layer of a eutectic mixture with the substrate or the layer of the ceramic material connected is characterized in that for forming the intermediate layer, a mixture of at least the metallic material, an oxide of the metallic material and Al ₂ O ₃ as a ceramic filler between the film or plate and the substrate or the layer of the ceramic material is brought. A method according to claim 1, characterized in that the mixture is provided as a powder or paste and brought between the film or plate and the substrate or the layer of the ceramic material. A method according to claim 1 or 2, characterized in that the mixture additionally contains at least one further ceramic material. A method according to claim 3, characterized in that the Al ₂ O _{3 is applied} as a coating on powder particles of the further ceramic material. A method according to claim 3 or 4, characterized in that the at least one further ceramic material is selected so that by this further ceramic material, the thermal expansion coefficient of the intermediate layer is closer to an average of the thermal expansion coefficient of the plate or film and the substrate or the layer as without this further ceramic material. Method according to one of claims 1 to 5, characterized in that the mixture is applied by means of screen printing or by means of a spraying technique on the plate or film and / or the substrate or the layer. Method according to one of claims 1 to 6 for the production of printed circuit boards or metallized substrates for electrical or electronic circuits. Composite material of a ceramic substrate or a ceramic layer and a metallic layer, which is produced according to one of claims 1 to 6. Composite material according to claim 8, characterized in that the metallic layer is patterned on the ceramic substrate or the ceramic layer.

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