Copper-ceramic substrate, copper precursor for producing a copper-ceramic substrate and process for producing a copper-ceramic substrate

Abstract

The present invention relates to a copper ceramic substrate incorporating a ceramic carrier, and a copper layer joined to a surface of the ceramic carrier, wherein the copper layer incorporates at least one first layer, which faces the ceramic carrier and has an average first grain size, and a second layer, which is arranged on the face of the copper layer facing away from the ceramic carrier and has an average second grain size, the second grain size being smaller than the first grain size.

Claims

1. A copper ceramic substrate, comprising: a ceramic carrier; and a copper layer joined to a surface of the ceramic carrier, wherein the copper layer comprises: at least one first layer; wherein the at least one first layer faces the ceramic carrier and has a first average grain size, and a second layer; wherein the second layer is arranged on a face of the copper layer facing away from the ceramic carrier and has a second average grain size, wherein the second average grain size is smaller than the first average grain size, and wherein: a) the first average grain size is in a range of from 250 μm to 1000 μm, and the second average grain size is less than 100 μm; or b) the first average grain is in a range of from 250 μm to 2000 μm, and the second average gram size is less than 150 μm.

2. The copper ceramic substrate according to claim 1, wherein the at least one first layer and the second layer of the copper layer are formed of at least two different copper materials.

3. The copper ceramic substrate according to claim 2, wherein the at least one first layer is formed of Cu-ETP.

4. The copper ceramic substrate according to claim 3, wherein the second layer is formed of Cu-OF or Cu-OFE.

5. The copper ceramic substrate according to claim 1, wherein the at least one first layer has a first proof stress, wherein the second layer has a second proof stress, and wherein the first proof stress is lower than the second proof stress.

6. The copper ceramic substrate according to claim 1, wherein the at least one first layer has a melting point of 1060° C. to 1085° C.

7. A method for producing a copper ceramic substrate, comprising: providing a ceramic carrier; providing at least one first layer; providing a second layer, wherein the at least one first layer and the second layer are formed of copper materials, joining the at least one first layer and the second layer together to form a copper semi-finished product, bonding the copper semi-finished product to a surface of the ceramic carrier to form a copper ceramic substrate, wherein the copper ceramic substrate comprises: a copper layer joined to the surface of the ceramic carrier, wherein the copper layer comprises: the at least one first layer; and wherein the at least one first layer faces the ceramic carrier and has a first average grain size; the second layer; wherein the second layer is arranged on a face of the copper layer facing away from the ceramic carrier, wherein the second layer has a second average grain size, wherein the second average grain size is smaller than the first average grain size, wherein a difference between the first average grain size and the second average grain size is achieved by: (i) joining the at least one first layer and the second layer together when the at least one first layer and the second layer have different average grain size; (ii) applying heat to the at least one first layer and the second layer after joining the at least one first layer and the second layer together, and prior to bonding the copper semi-finished product to the surface of the ceramic carrier; (ii) applying heat to the at least one first layer and the second layer during bonding the copper semi-finished product to the surface of the ceramic carrier; (i) and (ii); (i) and (iii); (ii) and (iii); or (i), (ii), and (iii), and wherein: a) the first average grain size is in a range of from 250 μm to 1000 μm, and the second average grain size is less than 100 μm; or b) the first average grain size is in a range of from 250 μm to 2000 μm, and the second average grain size is less than 150 μm.

8. The method according to claim 7, wherein joining the at least one first layer and the second layer together is accomplished via plating.

9. The method according to claim 7, wherein the second average grain size is approximately 50 μm.

10. The copper ceramic substrate according to claim 1, wherein the second average grain size is approximately 50 μm.

Description

(1) The invention is described in greater detail below with reference to a preferred embodiment. In the drawings, in detail:

(2) FIG. 1 shows a copper ceramic substrate according to the prior art;

(3) FIG. 2 shows a copper semi-finished product comprising two layers made of different copper materials; and

(4) FIG. 3 is a sectional view of a copper ceramic substrate according to the invention.

(5) Power modules are semiconductor devices of power electronics and are used as semiconductor switches. They contain a plurality of power semiconductors (chips) in a housing, which are electrically insulated from the heat sink. These are applied to a metallised surface of an electrically insulating plate (e.g. made of ceramic) by means of soldering or adhesive bonding, so as to ensure the heat output towards the base plate and also ensure the electrical insulation. The composite made of metallised layers and the insulating plate is referred to as a copper ceramic substrate and is created on an industrial scale by means of what is referred to as DCB technology (direct copper bonding).

(6) Contact between the chips is established by means of bonds that have thin bonding wires. Moreover, further assemblies which have different functions (e.g. sensors, resistors) can be present and integrated.

(7) To produce a DCB substrate, ceramic carriers (e.g. Al.sub.2O.sub.3, Si.sub.3N.sub.4, AlN, ZTA, ATZ) are joined to one another on the upper and lower faces thereof by copper layers in a bonding process. In preparation for this process, the copper layers can be oxidised (e.g. chemically or thermally) on the surface before being placed on the ceramic carrier, and can be subsequently placed on the ceramic carrier. The join is produced in a high temperature process between 1060° C. and 1085° C., a eutectic melt being produced on the surface of the copper layer, which melt produces a join to the ceramic carrier. For example, in the case of copper (Cu) on aluminium oxide (Al.sub.2O.sub.3), this join consists of a thin Cu—Al spinel layer.

(8) FIG. 1 shows a copper ceramic substrate 1 according to the prior art, which comprises a ceramic carrier 2 and two copper layers 3 and 4 held on the surface of the ceramic carrier 2.

(9) FIG. 3 shows a copper ceramic substrate 1 that has been developed according to the invention and comprises a ceramic carrier 2 and two copper layers 3 and 4. The two copper layers 3 and 4 in FIG. 3, which have been developed according to the invention, each comprise a first layer 5 and 6 that faces the ceramic carrier 2 and has a coarser microstructure. The first layers 5 and 6 are preferably those layers with which the copper layers 3 and 4 rest against the ceramic carrier 2, and which form the join to the ceramic carrier 2.

(10) The first layers 5 and 6 of the copper layers 3 and 4 are each covered, on the free outer face, by a second layer 7 and 8 that has a finer microstructure having an average finer grain size of less than 100 or 150 μm, preferably of approximately 50 μm. However, the first layers 5 and 6 of the two copper layers 3 and 4 have a coarser microstructure that has an average larger grain size of greater than 100 or 150 μm, preferably of approximately 250 to 1000 μm or 250 to 2000 μm. The microstructures of the copper layers 3 and 4 thus have, in the first layers 5 and 6 that face the ceramic carrier 2, a grain size that is on average 10 times greater than the grain size in the layers 7 and 8 that are directed outwards. The first layers 5 and 6 can be substantially thicker than the second layers 7 and 8 and form the base layers of the copper layers 3 and 4. The second layers 7 and 8 can be substantially thinner, have a thickness of approximately 50 to 100 μm, and form the free surfaces of the copper layers 3 and 4. As a result of the considerably greater thickness of the first layers 5 and 6 which have the larger grain size, the mechanical behaviour of the copper layers 3 and 4 is changed to such an extent that the copper layers 3 and 4 have, overall, a lower proof stress and thus a higher thermal fatigue resistance, while the second layers 7 and 8, which have the substantially finer microstructure, merely form the free surface.

(11) The copper layers 3 and 4 can, for example, be joined to the ceramic carrier 2 after the DCB method described at the outset such that the two first layers 5 and 6 of the two copper layers 3 and 4 that rest thereon are joined to the ceramic carrier 2 by means of an integral join in the respective surface edge zones 9 and 10 of the ceramic carrier 2. Since the two copper layers 3 and 4 in the first layers 5 and 6 have a considerably coarser microstructure that has a large grain size of 250 to 1000 μm or 250 to 2000 μm, they also have, as a result of the Hall-Petch relationship described above, a lower proof stress than in the region of the second layers 7 and 8 arranged on the outer faces, and therefore are joined to the ceramic carrier 2 at a higher thermal fatigue resistance than would be the case if they had the same grain size of 50 μm on this face as on the outer face. Thus, the copper layers 3 and 4 that are formed by means of the proposed design and have the larger grain size on the face of the join to the ceramic carrier 2 are specially designed for a high thermal fatigue resistance in the join between the surface edge zones 9 and 10. In contrast, the copper layers 3 and 4 on the outer faces thereof, as a result of the finer microstructure of the second layers 7 and 8 that have the finer grain size of 50 μm, are considerably more simply and precisely processed for introducing the conductor structure. Moreover, the layers on this face have a greater hardness, strength and proof stress, such that the lifetime of the copper ceramic substrate 1 can also be increased with respect to external influences. Furthermore, the finer-grained structure of the second layers 7 and 8 that form the surface is advantageous for joining wires.

(12) The different microstructure of the copper layers 3 and 4 in the different layers 5, 6, 7 and 8 can be achieved by means of a specifically undertaken heat treatment or by means of using two different copper materials or by means of a combination of the two measures. According to a preferred embodiment, the two copper layers 3 and 4 are produced by means of plating (such as roll-bonding) a layer of the copper material Cu-OF, preferably Cu-OFE, with a layer of the copper material Cu-ETP to form a copper semi-finished product 11 and 12, which is shown in FIG. 2. The first layer 5, 6, formed by the Cu-ETP, and the second layer 7, 8, formed by the Cu-OF, here have an identical or at least comparable grain size. During the DCB method, the pre-oxidised copper semi-finished product 11 or 12 is placed on the ceramic carrier 2, for example, and then heated to the process temperature of 1060° C. to 1085° C. In so doing, the Cu oxide in the first layer 5, 6, which in this case is formed by the Cu-ETP, melts and forms the joins to the ceramic carrier 2 in the surface edge zones 9 and 10. As a result of the application of heat and the different recrystallisation behaviour of the two copper materials, the structure is changed to such an extent that the particles in the Cu-ETP are subsequently coarse, while the Cu-OF or the Cu-OFE has a finer microstructure.

(13) Cu-OF or Cu-OFE and Cu-ETP are highly conductive Cu materials and have a conductivity of greater than or equal to 58 MS/m. However, materials which have a lower conductivity are also conceivable. Furthermore, the two Cu materials can also be joined to one another by means of other joining methods, such as welding, soldering, stapling, adhesive bonding or additive manufacturing methods. Furthermore, the copper layers 3 and 4 can, if required, also be supplemented by further Cu materials or layers, provided that the material properties of the copper layers 3 and 4 are intended to be further refined.

(14) The two copper layers 3 and 4 are preferably prefabricated as copper semi-finished products 11 and 12, in each case by plating the two Cu materials. After bonding, the copper semi-finished products 11 and 12 already have, as a result of the proposed use of different Cu materials and applying heat during bonding, a microstructure on one face that has a finer grain size of approximately 50 μm and a microstructure on the other face that has a larger grain size of 250 to 1000 μm. In this case, the bonding simultaneously deliberately constitutes a heat treatment, during which the grains of the first layers 5 and 6 of the copper layers 3 or 4, which face the ceramic carrier 2, increases further in size, and this in turn is positive in the sense of a further increased thermal fatigue resistance of the copper layers 3 and 4 and in particular in the region of the joins 9 and 10 to the ceramic carrier 2. At the same time, the heat treatment does not lead to a noteworthy increase in the grain size in the layers 7 and 8 of the copper layers 3 or 4 or of the copper semi-finished product 11 and 12 that face away from the ceramic carrier 2, such that the properties of the copper layers 3 and 4 on this face are not disadvantageously changed.

(15) According to one embodiment, two different Cu materials are joined to one another by means of plating, such that the high temperature properties in the finished material can be specifically set such that a coarse structure having low proof stress is produced in the copper layers 3 and 4 during the application of heat on the faces facing the ceramic carrier 2 and such that a finer structure having the required surface properties is produced on the free surface. In so doing, further layers which also have a finer structure can also be present between the first layers 5 and 6, which have the coarser structure, and the ceramic carrier 2, provided that this is advantageous for the specific application. However, the main advantage of the reduction in the formation of fissures and prevented delamination under thermal fatigue stress remains, since the first layers 5 and 6 of the copper layers 3 and 4 in this case also form a core which has a reduced proof stress and a thereby increased thermal fatigue resistance.

(16) Furthermore, the bonding is also used in this case, preferably alongside the creation of the join, as a heat treatment, by means of which it is particularly simple to achieve the different grain sizes proposed according to the invention on the two faces of the copper layers 3 and 4, it being possible to further improve the effect by using the two different copper materials.

(17) The copper layers 3 and 4 are preferably prefabricated as copper semi-finished products 11 and 12, which are produced by plating the two proposed copper materials. In so doing, the first layers 5 and 6, which are facing the ceramic carrier 2 and are formed by the Cu-ETP, are designed so as to be substantially thicker and so as to form a type of carrier function for the substantially thinner second layers 7 and 8, formed by the Cu-OF.

(18) The copper semi-finished products 11 and 12 can have a thickness of 0.1 to 1.0 mm, are placed in large dimensions on the ceramic carrier 2, and are joined to the ceramic carrier 2 by means of the DCB method. Subsequently, the large-surface-area copper ceramic substrate 1 is cut into smaller units and further processed.

(19) Alongside the improved copper ceramic substrate 1 and the copper sheets which are prefabricated as copper semi-finished products 11 and 12, the invention also provides a preferable cost-effective method for producing the copper ceramic substrate 1. In this method, the proposed copper ceramic substrate 1 is preferably produced by means of a heat treatment, by means of which the different grain sizes in the two layers 5 and 6 or 7 and 8 are automatically adjusted. In this case, the copper layers 3 and 4 can be subjected to the heat treatment before being joined to the ceramic carrier 2, or the application of heat can be used during the bonding method in order to influence the microstructure. Furthermore, the copper layers 3 and 4 can also be put together by plating two different Cu materials which already have a different microstructure or which then form the different microstructures during the heat treatment.

Embodiment 1

(20) Copper ceramic substrate (1) comprising

(21) a ceramic carrier (2), and

(22) a copper layer (3, 4) joined to a surface of the ceramic carrier (2), characterised in that

(23) the copper layer (3, 4) comprises at least one first layer (5, 6), which faces the ceramic carrier and has an average first grain size, and a second layer (7, 8), which is arranged on the face of the copper layer (3, 4) facing away from the ceramic carrier (2) and has an average second grain size,

(24) the second grain size being smaller than the first grain size, and

(25) the first layer (5, 6) having an average grain size of greater than 100 μm, preferably approximately 250 to 1000 μm, and

(26) the second layer (7, 8) having an average grain size of less than 100 μm, preferably approximately 50 μm, or

(27) the first layer (5, 6) having an average grain size of greater than 150 μm, preferably approximately 250 to 2000 μm, and

(28) the second layer (7, 8) having an average grain size of less than 150 μm, preferably approximately 50 μm.

Embodiment 2

(29) Copper ceramic substrate (1) according to Embodiment 1, characterised in that

(30) the first and the second layer (5, 6, 7, 8) of the copper layer (3, 4) are formed by at least two different copper materials.

Embodiment 3

(31) Copper ceramic substrate (1) according to Embodiment 2, characterised in that

(32) the copper material of the first layer (5, 6) is Cu-ETP.

Embodiment 4

(33) Copper ceramic substrate (1) according to either Embodiment 2 or Embodiment 3, characterised in that

(34) the copper material of the second layer (7, 8) is Cu-OF, more preferably Cu-OFE.

Embodiment 5

(35) Copper ceramic substrate (1) according to any of the preceding Embodiments, characterised in that

(36) the first layer (5, 6) has a lower proof stress than the second layer (7, 8).

Embodiment 6

(37) Copper ceramic substrate (1) according to any of the preceding Embodiments, characterised in that

(38) the first layer (5, 6) has a melting point of 1060° C. to 1085° C.

Embodiment 7

(39) Copper semi-finished product (11, 12) for producing a copper ceramic substrate (1), characterised in that

(40) the copper semi-finished product (11, 12) comprises at least one first layer (5, 6) that has an average first grain size and a second layer (7, 8) that has an average second grain size, the two average grain sizes being different,

(41) the first layer (5, 6) having an average grain size of greater than 100 μm, preferably approximately 250 to 1000 μm, and the second layer (7, 8) having an average grain size of less than 100 μm, preferably approximately 50 μm, or

(42) the first layer (5, 6) having an average grain size of greater than 150 μm, preferably approximately 250 to 2000 μm, and

(43) the second layer (7, 8) having an average grain size of less than 150 μm, preferably approximately 50 μm.

Embodiment 8

(44) Copper semi-finished product (11, 12) according to Embodiment 7, characterised in that

(45) the two layers (5, 6, 7, 8) are formed by different copper materials.

Embodiment 9

(46) Copper semi-finished product (11, 12) for producing a copper ceramic substrate (1), characterised in that

(47) the copper semi-finished product (11, 12) comprises a first layer (5, 6) made of a first copper material and a second layer (7, 8) made of a second copper material,

(48) the first and the second copper material being designed so as to have different grain sizes after an application of heat.

Embodiment 10

(49) Copper semi-finished product (11, 12) according to either Embodiment 8 or Embodiment 9, characterized in that

(50) the two different copper materials are joined to one another by means of plating.

Embodiment 11

(51) Copper semi-finished product (11, 12) according to any of Embodiments 8 to 10, characterized in that

(52) the two different copper materials are Cu-OF, preferably Cu-OFE, and Cu-ETP.

Embodiment 12

(53) Copper semi-finished product (11, 12) according to any of Embodiments 7 to 11, characterized in that

(54) the copper semi-finished product (11, 12) undergoes different heat treatments in the two layers (5, 6, 7, 8).

Embodiment 13

(55) Method for producing a copper ceramic substrate (1) having the features of any of Embodiments 1 to 6, characterised in that

(56) the different grain sizes of the two layers (5, 6, 7, 8) of the copper layer (3, 4) are achieved by applying heat during a bonding method for joining the copper layer (3, 4) to the ceramic carrier (2).

Embodiment 14

(57) Method according to Embodiment 13, characterised in that the copper layer (3, 4) is formed by a copper semi-finished product (11, 12) according to Embodiment 7 or according to any of Embodiments 8 to 12 when dependent on Embodiment 7.