GB2231524A

GB2231524A - Metal and ceramic-substrate bonding

Info

Publication number: GB2231524A
Application number: GB9003422A
Authority: GB
Inventors: Andrew Richard Hyde; Graham Partridge
Original assignee: English Electric Co Ltd; GEC Alsthom Ltd
Current assignee: English Electric Co Ltd; Alstom UK Ltd
Priority date: 1989-02-15
Filing date: 1990-02-15
Publication date: 1990-11-21
Anticipated expiration: 2010-02-15
Also published as: GB2231524B; GB8903425D0; GB9003422D0

Abstract

A method of bonding a layer of metal to a ceramic substrate, particularly say, copper to alumina or zirconia, in which the substrate or the metal or both are coated with a solution of oxygen-rich metal salt, eg. copper perchlorate or copper nitrate. The copper layer is assembled to the alumina with the intervening coating and the assembly heated in an inert atmosphere. The copper salt breaks down at about 600 DEG to 700 DEG C leaving a deposit of copper oxide on the ceramic and releasing oxygen which oxidises the copper layer. The two layers of copper oxide then amalgamate in a common melt and provide a firm bond. This method provides a controllable local oxidising atmosphere at the contacting surfaces while the whole assembly is maintained in an inert atmosphere. The salt is preferably a salt of the same metal as the metal which is being bonded. The bonding of the metal to the ceramic substrate may be followed by etching of the metal to produce electric circuitry.

Description

Metal - Ceramic-Substrate Bonding This invention relates to a method of bonding a metal to a ceramic or glass-ceramic substrate and to components formed by such method.

Substrates used in microwave and hybrid circuitry comprise a sheet of ceramic (often alumina) or glass-ceramic (hereafter referred to as ceramic) on which is provided metallic layers and circuitry.

The ceramic provides the necessary insulating support and also enables the circuitry to operate at higher temperatures and under more severe environmental conditions than can be withstood by insulating boards based on glass reinforced plastic.

Various ways exist of providing the combination of metallic circuit elements on to the ceramic substrate. These include application of thick film conductor inks by means of screen printing followed by firing; application of thin film conductors by means of evaporation through a suitable mask on to the substrate; and application of an overall layer of metal on to the ceramic followed by etching of the metal to delineate the required circuitry. It is to this latter method that the present invention is directed, this method being particularly advantageous where relatively broad circuit lines are required, capable of carrying high electric currents. The invention should not, however, be considered as limited to any particular surface dimensions of metal layer.

Previous work in this field, by other researchers, has led to the development of "eutectic" bonding in which copper is bonded to the ceramic by making use of the eutectic formed between copper and copper oxide at a temperature of 1065"C. This process is illustrated in, for example, US Patent 3,994,430, assigned to General Electric USA.

Various ways exist of achieving this type of bond but in general terms it is necessary to develop a thin layer of copper oxide on the contact face of the copper prior to the firing of the copper in juxtaposition with the ceramic. The firing is carried out, usually in an inert atmosphere such as nitrogen, although reducing atmospheres may be employed. This prevents excessive oxidation of the exposed surface of the copper. During the firing the copper/copper oxide eutectic formed at the interface wets and achieves bonding to the ceramic.

An object of the present invention is to provide an improved method of generating the bond between the metal and the ceramic substrate and controlling the oxidation/reduction conditions in the interface region. A further object is to facilitate the carrying out of the process on a through-put basis thus allowing high production rates to be achieved.

According to the present invention, in a method of bonding a metal surface to a ceramic substrate the ceramic substrate and/or the metal surface is coated with a solution of a salt or mixture of salts, the metal surface is placed in contact with the coated ceramic and heated in an inert atmosphere to the eutectic temperature of the metal and the oxide of the metal so as to cause; breakdown of the salt, producing an oxide of the salt metal on the ceramic; a locally oxygen-rich atmosphere; oxidation of the metal surface; and a common melt between the two oxide layers providing a bond between the ceramic substrate and the metal surface; the bonded metal/substrate being cooled in an inert atmosphere.

The salt is preferably a salt of the same metal as that of the metal surface.

The metal may be copper and the salt an oxygen-rich salt such as copper perchlorate or copper nitrate.

The metal surface may be plated on to a component of different metal where the latter metal and the salt are not compatible.

According to a feature of the invention, a method of bonding a copper layer to a ceramic substrate includes the steps of: (a) applying a coating of copper perchlorate or copper nitrate to the ceramic substrate; (b) assembling the copper layer in contact with the ceramic substrate; (c) heating the assembly in an inert atmosphere to a temperature in the region of 10700C; (d) maintaining said temperature for between 1 minute and two hours; and (e) cooling said assembly to room temperature in said inert atmosphere; whereby the copper salt is broken down into copper oxide and oxygen to produce a copper oxide layer on the ceramic, a locally oxygen-rich atmosphere which oxidises the copper surface to copper oxide, and a common melt of the copper oxide layers to provide a bond between the copper layer and the ceramic substrate.

According to another aspect of the invention, a component comprises a metal layer bonded to a ceramic substrate, formed by a method as aforesaid.

A method of bonding a metal to a ceramic substrate, in accordance with the invention, will now be described, by way of example.

In general the process comprises the following steps: (a) Cleaning the ceramic and metallic components in appropriate cleaning media, eg nitric acid, halogenated solvents.

(b) Application of solutions of oxygen rich copper salts, for example, copper perchlorate or copper nitrate on to the ceramic component. These may be applied by means of dipping, spin-coating, printing or spraying. The strength of the solution determines the quantity of solids left on the surface of the ceramic component following drying.

(c) The coated ceramic and copper components are assembled and fired through a continuous throughput furnace in an atmosphere of nitrogen to a temperature in the region of 10700C (say 10700Cf150C) which is maintained for up to 2 hours followed by cooling to room temperature in the blanketing nitrogen atmosphere.

During this process a number of chemical reactions occur.

Firstly, the copper perchlorate or copper nitrate breaks down at about 600-700"C according to the following reactions: Cu(C104)2 4 CuO + 2C102 + 3/202 (1) 2Cu(N03)2 -3 Cu2O + 4NO:, + /2 2 (2) This provides a layer of copper oxide on the surface of the ceramic. At the same time, the oxidising atmosphere generated in the space between the copper and the ceramic causes oxidation of the copper contact face to occur under the locally controlled oxidising conditions.

Cu + (0) - > CuO (3) Reaction between copper and copper (II) oxide yields copper (I) oxide.

CuO + Cu - > Cu2O (4) By the time the eutectic temperature is reached the two layers of copper oxide, ie CuO and/or Cu2O on the ceramic and CU20 and/or CuO on the copper which have been formed separately but are in contact with each other, result in a common melt. It is believed that this aspect enhances the formation of a strong and reliable bond.

It is important to note that the materials are heated to the eutectic temperature in an atmosphere which is intentionally locally rich in oxygen and so is not inert at the place where the reaction is taking place. The process enables controlled oxidation levels to be achieved in the interspace between copper and ceramic, at the very point and time at which it is needed, and so obviates the need for either preoxidation of the copper or provision and control of prescribed levels of oxygen in the surrounding atmosphere. Greater and easier control of the bonding process can thus be achieved leading to greater reliability of the bond.

(d) As a final stage, the circuitry in the copper is achieved in a standard manner by means of masking appropriate areas and etching away other portions, using, for example, ferric chloride solution.

A number of specific examples of the method, providing a variety of bonded components, is now given.

1. A 50 x 50mm2 96% Al203 substrate was dip coated in a copper (II) nitrate solution of concentration 179 of solute to 100ml of water. The coated substrate was oven dried (80"C:30minutes) whereupon precleaned copper substrates 49 x 49mm2 of 0.5mm thickness were applied to either face of the coated Al203. The sandwich was bonded by heat-treatment through a nitrogen atmosphere continuous through-put furnace reaching a maximum temperature of 1070"C maintained for 4 minutes.

2. A 150mm diameter 96% Al203 substrate dip coated in a copper (II) perchlorate solution of concentration 28g of solute of 100ml of water. The coated substrate was oven dried (1000C:15 minutes) whereupon cleaned copper substrates of 147mm diameter and 3mm thickness were applied to either face of the coated Al203. The sandwich was bonded by heat-treatment through a nitrogen atmosphere continuous through-put furnace of 1070"C maintained for 3 minutes.

3. Two 25mm x 75mm2 96% Al203 substrates were dip coated in a copper (11) perchlorate solution of concentration 20g of solute to 100ml of water. The coated substrates were air dried whereupon precleaned copper substrates 24 x 73mm2 of 0.5mm thickness were applied to the faces of the alumina to yield a copper-alumina-copper-alumina-copper sandwich. The sandwich was bonded by heat-treatment as described in example 2.

4. A 50 x 50mm2 At203 substrate was dip coated and dried as described in example 1. Precleaned copper substrates 49 x 49mm2 of 0.5mm thickness were applied to either face of the coated Al203.

Additional clean (uncoated) Al203 substrates were applied to yield an alumina-copper-alumina-copper-alumina sandwich. The sandwich was bonded by heat-treatment as described in example 1. Copper oxide was transported through the copper during the heat-treatment process to bond the uncoated At203 substrates and yield a multi-sandwich structure.

5. One face of a 20 x 100mm2 96% Al203 substrate was spin coated with a copper (II) nitrate solution of concentration 159 of solute to 100ml of water. A disc of precleaned copper of iOmm diameter and 0.4mm thickness was applied to the surface of the coated Al203. An 11mm disc of-zircon spin-coated with copper (II) nitrate was placed upon the copper disc. The sandwich was bonded by heat-treatment as described in example 1.

The process is not limited to bonding copper to alumina but may be used with any suitable ceramic substrate, eg zirconia, glass-ceramic. Other metallic elements may be used where suitable chemical precursors are available, or alternatively, copper salts may be used, although, if the oxides of the two metals, ie copper oxide from the salt solution and the oxide of the different metal component, are not compatible, pre-plating of the second metal with copper may be necessary to achieve the desired reaction.

As an alternative to coating solely the ceramic substrate it is possible to form strong bonds by coating either the copper or the copper and ceramic with the copper oxide precursor solutions, the resulting materials being heat-treated in a similar manner.

Claims

1. A method of bonding a metal surface to a ceramic substrate wherein the ceramic substrate and/or the metal surface is coated with a solution of a salt or mixture of salts, said metal surface is placed in contact with the coated ceramic and heated in an inert atmosphere to the eutectic temperature of the metal and the oxide of the metal so as to cause; breakdown of the salt, producing an oxide of the salt metal on the ceramic; a locally oxygen-rich atmosphere; oxidation of said metal surface; and a common melt betwen the two oxide layers providing a bond between the ceramic substrate and the metal surface; the bonded metal/substrate being cooled in an inert atmosphere.

2. A method according to Claim 1, wherein said salt is a salt of the same metal as said metal surface.

3. A method according to Claim 2, wherein said metal is copper and said salt is an oxygen-rich salt.

4. A method according to Claim 3, wherein said salt is copper perchlorate or copper nitrate.

5. A method according to any preceding claim wherein said substrate is alumina.

6. A method according to Claim 3 or Claim 4, wherein said metal surface is plated on to a component of different metal.

7. A method of bonding a copper layer to a ceramic substrate including the steps of: (a) applying a coating of copper perchlorate or copper nitrate to the ceramic substrate; (b) assembling the copper layer in contact with the ceramic substrate; (c) heating the assembly in an inert atmosphere to a temperature in the region of 1070"C; (d) maintaining said temperature for between 1 minute and 2 hours; and (e) cooling said assembly to room temperature in said inert atmosphere; whereby the copper salt is broken down into copper oxide and oxygen to produce a copper oxide layer on the ceramic, a locally oxygen-rich atmosphere which oxidises the copper surface to copper oxide, and a common melt of the copper oxide layers to provide a bond between the copper layer and the ceramic substrate.

8. A method of bonding a copper layer to a ceramic substrate substantially according to any of the examples hereinbefore described.

9. A component comprising a metal layer bonded to a ceramic substrate, formed by a method according to any preceding claim.