EP1028929A1

EP1028929A1 - Metal-aluminium-nitride assembly with rare earth nitride present in the interface to ensure heat transfer

Info

Publication number: EP1028929A1
Application number: EP99941733A
Authority: EP
Inventors: Thierry Baffie; François Saint-Antonin
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1998-09-11
Filing date: 1999-09-10
Publication date: 2000-08-23
Also published as: FR2783185B1; JP2002524388A; WO2000015578A1; FR2783185A1

Abstract

The invention concerns the assembling of a first element comprising aluminium nitride and a second metal or metal alloy element, wherein the interface between the two elements comprises at least a simple or complex rare earth, scandium and/or yttrium nitride. Said assembly can be obtained by soldering providing between the elements a soldering sheet made of an alloy comprising at least a rare earth, scandium and/or yttrium, for example a Cu-Y, Cu-Dy or CU-Ag-Dy alloy.

Description

METAL-NITRU E ASSEMBLY OF ALUMINUM, WITH PRESENCE OF RARE EARTH (S) NITRIDE (S) AT THE INTERFACE TO ENSURE

THERMAL TRANSFER

DESCRIPTION

Technical area

The present invention relates to an assembly between a first element comprising aluminum nitride and a second element of metal or metal alloy, which ensures good heat transfer between the two elements.

Such an assembly, the particularity of which is to conduct heat well between the two materials, can be used in various fields where high heat transfer is necessary. It can be used for example in

1 microelectronics and power electronics industry, or in any system requiring heat removal between a metal element and a ceramic aluminum nitride.

Indeed, the increasing increase in the density of the components, in particular of power, on the circuits, generates heat flows more and more important. The interfaces existing at present between metallic and ceramic elements do not allow a sufficient evacuation of this heat because the interface between the elements still constitutes a thermal barrier. Therefore, complex cooling systems are required. State of the art

There are known methods of assembly between metallic elements and elements made of aluminum nitride by means of titanium nitride or zirconium. Thus, the document: TOMSIA A. P. et al,

Reactions and Microstructure at Selected Ceramic / Metal Interfaces - Mater. Manuf. Process. 9, (3), 547-561, 1994 [1], illustrates the production of a solder between silver-copper alloys and a ceramic substrate based on aluminum nitride, in which one forms at the interface between the ceramic and metal a titanium or zirconium nitride. In this document, we mainly study the wetting of the ceramic by the silver-copper alloy containing titanium and not the thermal transfer at the interface.

The document NICHOLAS MG et al, Some Observations on the Wetting and Bonding of Nitride Ceramics, J. Mater. Sci. 25, (6), 2679-2689, 1990 [2], also shows that brazing alloys based on silver, copper and titanium wet ceramics based on aluminum nitride and that the bond obtained which contains titanium nitride is good. As before, this document is not concerned with heat transfer at the interface. Document JP-A-05 271 828 [3] illustrates the metallization of an aluminum nitride substrate by means of a composition comprising copper and 5 to 25% by weight of rare earths, which makes it possible to obtain a good bond between the metallization layer and the aluminum nitride substrate. In this document, the metallization layer is formed from copper powders and a rare earth, which during application to air inevitably leads to the formation of rare earth oxide having a very low thermal conductivity. Such a connection cannot therefore ensure good thermal transfer at the interface.

Document JP-A-05 171 317 [4] also illustrates the metallization of an aluminum nitride substrate by means of a copper alloy, with 5 to 25% by weight of rare earth and 0.5% to 10% by weight of another metal chosen from Fe, Co and Ni. A strong bond is thus obtained between the aluminum nitride substrate and the metallization layer with a high resistance to oxidation and to heat. As before, this document is not concerned with ensuring good thermal transfer at the interface between a ceramic element and a metallic element.

Thus, the previous documents and the literature relating to the metal-ceramic assembly do not address the problem of heat transfer at the interface between the ceramic and the metal or the metal alloy. However, in many applications, these assemblies must allow significant heat dissipation. This is particularly the case for electronic applications for which alumina is the most widely used ceramic to date. Alumina has a thermal conductivity of 25 W / mK while aluminum nitride has a thermal conductivity of 170 to 200 W / mK However, so far the gain provided by aluminum nitride on alumina is much lower than expected, due to the poor thermal conduction of the metal-aluminum nitride interface. This can be explained by the presence at the interface of compounds poor thermal conductors and many faults.

In the case of interfaces comprising titanium nitride or zirconium nitride such as those of references [1] and [2], these nitrides do not make it possible to control the heat transfer between the two elements. Indeed, these nitrides have a very wide range of existence, as it appears in the document: MASSALSKI T.B., Binary Alloy Phase Diagrams, 2nd Edition, Vol. 1, pages 1084,

2714, 2707 and 2717, 1990, [5]. According to the document :

KOSOLAPOVA T. Ya, Handbook of High Temperature

Compounds: Properties, Production, Application,

Hemisphere Publishing Corporation, New York, p.179-180 and p. 383, 1990 [6], the formulas of these titanium and zirconium nitrides are TiN _! _ _x with x varying from 0 to 0.49 and ZrN _! _ _x with x varying from 0 to 0.2. The existing data from references [6] and [7] on the thermal conductivities of titanium and zirconium nitrides show that these vary significantly depending on the nature of the nitride formed.

Thus, for TiN _0, _{from 83} to 27 ° C, the thermal conductivity is 12.5 W / mK; for TiN _0.9 , it is 30.5 W / mK and for TiN of

41.8 W / m.K. Thus, the thermal conductivity increases with the nitrogen content of the nitride.

In the case of zirconium, as appears from the document: Samsonov GV and VINITSKI IM Handbook of Refractory Compounds, Plenum Press, New York, p. 195, 1980 [7], the thermal conductivity of ZrN _{0 nitride, 92} is 28 2 W / m. K at 27 ° C. The problem posed by an interface produced from these nitrides is therefore that it does not allow a control of the thermal conductivity due to the possibility of forming a nitride with variable stoichiometry.

The present invention specifically relates to an assembly between aluminum nitride and metal or metal alloy, in which the composition of the interface can be controlled to obtain a high thermal transfer between the aluminum nitride element and the element. metallic or metallic alloy.

Statement of the invention

Also, the subject of the invention is an assembly of a first element comprising aluminum nitride and a second element of metal or metal alloy, in which the interface between the two elements comprises at least one simple or complex nitride. rare earth, scandium and / or yttrium.

In this assembly, the interface is made of a simple or complex nitride of rare earth, scandium and / or yttrium, which has a high and controllable thermal conductivity because the composition of the rare earth nitrides cannot vary in proportions important. Indeed, conventionally, the rare earth nitrides are defined by the formula rNi_ _x with x = 0 to 0.1.

In this assembly, the first element based on aluminum nitride can be made of a material chosen from polycrystalline aluminum nitride, monocrystalline aluminum nitride, an aluminum nitride-metal composite material or aluminum nitride-ceramic comprising at least 40% by volume of aluminum nitride.

In these composites, the metal can be, for example, molybdenum, and the ceramic can be, for example, titanium diboride TiB ₂ .

When the first element is made of monocrystalline or polycrystalline aluminum nitride, it may be a solid part or a deposit made on another support such as silicon. The deposition is advantageously carried out by vapor deposition.

In the assembly of the invention, the second element is made of metal or a metal alloy which is a good conductor of heat. This second element can advantageously be made of copper or a copper alloy, for example a copper alloy containing at least one precious metal chosen from Ag, Pt, Pd and Au.

The rare earth nitride included in the interface between the two elements can be a nitride of one or more of the rare earths belonging to the series La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. It is also possible to use scandium nitride and / or yttrium nitride.

The assembly between the two elements can be carried out: by simple brazing, reactive or using hydrides, or

- by metallization (chemical or physical vapor deposition). Also, the invention also relates to a method of assembling a first element comprising aluminum nitride and a second element of metal or metal alloy. According to a first embodiment, the brazing technique is used, and the method comprises the following steps: a) placing between the two elements to be assembled a sheet of brazing of a metal alloy comprising at least one metal chosen from rare earths, scandium and yttrium, and b) brazing the assembly thus obtained at a temperature of 700 to 1500 ° C or at a temperature between 20 and 100 ° C above the melting temperature of the alloy of the solder sheet, possibly in a nitrogen atmosphere.

In this process, the metal alloy of the sheet is preferably a metal alloy compatible with the second element, to which the metal or metals chosen from rare earths, scandium and yttrium are added. When the second element is made of copper or a copper alloy, the metal alloy of the sheet is preferably a copper alloy comprising at least one metal chosen from rare earths, scandium and yttrium, and optionally a metal or several precious metals chosen from Ag, Au, Pt and Pd, and which may contain up to 15% by weight of indium. Generally, the metal alloy brazing sheet comprises up to 37% by weight of the metal belonging to the rare earth family, scandium and yttrium, and preferably 0.7 to 17% by weight. The precious metal content may vary from

0 to 72% by weight. In this process, the formation of rare earth oxides and aluminum is avoided by brazing in an oxygen-free atmosphere.

Indeed, the presence of oxygen in the interface is prohibitive because it inevitably leads to a decrease in the thermal conductivity of the assembly by the formation of rare earth oxides and / or aluminum which have low thermal conductivities. To avoid the presence of oxygen, it is important to produce the elements to be assembled, the solder sheet and the soldering in an oxygen-free atmosphere in order to have clean interfaces. The metals used must be very pure and the aluminum nitride element can be polished or chemically treated before assembly to remove any layer of alumina on the surface.

The brazing sheet used in this first embodiment of the process of the invention can be prepared from metals by melting the desired brazing alloy which is then cold rolled in the form of a sheet. The brazing alloy can be produced by induction melting in a cold crucible under a very good vacuum and cooled quickly. One can also use other techniques such as roll quenching to make these alloys.

The thickness of the solder sheet is generally 20 to 200 μm. It is also possible to produce the assembly according to the invention using a technique of depositing the rare earth or the rare earth nitride on the first element of the assembly. Also, according to a second embodiment of the method of the invention, it comprises the following steps: a) depositing on the first element at least one metal or metallic nitride chosen from rare earths, scandium, yttrium and their nitrides, under a nitrogen-based atmosphere, b) subjecting the first element thus coated to a heat treatment at a temperature of 1000 to 1900 ° C. to react the deposit with aluminum nitride, optionally in an atmosphere d nitrogen, c) placing the second element above the element thus treated so that it is in contact with the deposit, and d) optionally subjecting the assembly to a second heat treatment at a temperature of 700 to 1500 ° C. According to a third embodiment of the process of the invention, it comprises the following steps: a) depositing on the first element at least one metallic nitride chosen from the rare earth, yttrium and scandium nitrides, b) placing the second element to be assembled in contact with the deposit thus formed, and c) optionally subjecting the assembly thus obtained to a heat treatment, at a temperature of 700 to 1500 ° C.

In these last two embodiments, a deposition technique is used to form the solder and the last step corresponds to soldering.

In the second embodiment, the assembly interface is formed in two stages, the temperature of the first step being chosen as a function of the melting temperature of the rare earth used to react the deposit with aluminum nitride. In the third embodiment, the rare earth nitride is applied directly to the interface in a single step.

In these latter two embodiments, the deposition and any thermal treatment (s) are carried out under an oxygen-free atmosphere to avoid the formation of oxide which would adversely affect the thermal conductivity of the interface. . As before, the presence of oxygen on the elements to be assembled is also avoided. The deposition of metals belonging to the rare earth family, of yttrium and scandium and / or of their nitrides can be carried out by conventional techniques such as chemical or physical vapor deposition. In the last two embodiments described above, the second element can be placed above the deposit, either in solid form, or by directly forming this element above the deposit by depositing the metal or the metal alloy forming this second element.

In the case of copper or copper alloys, the deposition technique can be a conventional technique such as vapor deposition.

In the case where the second element is formed directly by deposition on the assembly, it is generally not necessary to carry out step d) or c) of heat treatment. With the assembly methods described above, the interface formed between the first element and the second element of the assembly allows thermal transfer from one to the other with a minimum of losses, ensuring a strong transmission and minimal heat reflection.

In the case where the second element is made of copper or a copper alloy, the interface generally has the following structure: by moving from the second element to the first element, there is successively copper or its alloy, possibly a zone composed mainly copper and copper / rare earth intermetallic, a layer of rare earth nitride and aluminum nitride. This assembly has the advantage of conducting heat well, which allows good thermal conduction of the metal such as copper to aluminum nitride. This is achieved first by the crystallographic compatibility of the materials which have small differences in lattice parameters, which allows good accommodation of the mechanical stresses due to the difference in coefficient of thermal expansion of the copper or the copper alloy. and rare earth nitride, on the one hand, and rare earth nitride and aluminum nitride, on the other hand.

Copper has a thermal conductivity of 400 W / mK and that of AIN is in practice from 170 to 200 W / mK The fact that the rare earth nitride formed at the interface has a fixed stoichiometry, that is to say that it exists over a very narrow range of composition unlike titanium nitride and nitride of zirconium, contributes to the good thermal conductivity of the assembly.

Other characteristics and advantages of the invention will appear better on reading the following examples, given of course by way of non-limiting illustration.

Detailed description of the embodiments

Example 1

In this example, a copper plate is assembled with a polycrystalline aluminum nitride substrate.

For this assembly, the brazing process is used by placing between the aluminum nitride substrate and the copper plate a sheet of copper-yttrium alloy of approximately 150 μm in thickness comprising 5.5% by atom ( 7.5% by weight) of yttrium. The copper plate is placed above the assembly and a molybdenum weight is placed above this plate which ensures mechanical contact between the substrate, the solder sheet and the copper plate.

The assembly is then introduced into a secondary vacuum oven, which has been swept beforehand by an inert gas in order to remove all traces of oxygen and water, and the assembly is heated to 1000 ° C. for 30 minutes.

This produces an assembly of the copper plate and the aluminum nitride substrate which has a high thermal conductivity. Example 2

In this example, a polycrystalline aluminum nitride substrate and a copper plate are assembled following the same procedure as in Example 1, but a sheet of copper alloy and dysprosium comprising 4 is used for the brazing. , 1% by atom (9.84% by weight) of dysprosium. The alloy is pre-prepared by quenching on a roller and is in the form of a sheet 40 μm thick. This sheet is cleaned in an ultrasonic bath with alcohol, then with acetone.

The aluminum nitride substrate was polished to the micron and cleaned in an alcohol and acetone ultrasonic bath just before the assembly was carried out. Likewise, the copper plate is mechanically etched to overcome any layer of surface oxide.

As in Example 1, the aluminum nitride substrate, the Cu-Dy alloy sheet and the copper plate are stacked, then the assembly is brazed in a secondary vacuum oven at a temperature at 1000 ° C for 30 minutes. An assembly is thus obtained with a high thermal conductivity.

Example 3

In this example, an aluminum nitride substrate obtained by vapor deposition and a copper plate are assembled using a copper-silver-dysprosium alloy sheet comprising 57.5% of silver and 4.1% dysprosium atom. The sheet alloy is pre-prepared in a cold crucible, then a part is rolled to make a sheet about 150 μm thick.

The copper-silver-dysprosium alloy sheet with a thickness of approximately 150 μm and the copper plate are placed above the aluminum nitride substrate, and a weight of molybdenum is placed over it to ensure mechanical contact between the elements. Brazing is carried out at 1000 ° C for 30 minutes, in a secondary vacuum oven as in the previous examples. The assembly obtained has good heat transfer properties.

References cited

[1]: TOMSIA A. P. et al, Reactions and Microstructure at Selected Ceramic / Metal Interfaces - Mater. Manuf. Process. 9, (3), 547-561, 1994. [2]: NICHOLAS M. G. et al, Some Observations on the Wetting and Bonding of Nitride Ceramics, J. Mater. Sci. 25, (6), 2679-2689, 1990.

[3]: JP-A-05 271 828.

[4]: JP-A-05 171 317

[5]: MASSALSKI T.B., Binary Alloy Phase Diagrams, 2nd Edition, Vol. 1, 2 and 33, p. 1084, 2707, 2714 and 2717, 1990.

[6]: KOSOLAPOVA T. Ya, Handbook of High Temperature Compounds: Properties, Production, Application, Hémisphère Publishing Corporation, New York, p.179-180 and p. 383, 1990.

[7]: Samsonov G.V. and VINITSKI I.M. Handbook of Refractory Compounds, Plenum Press, New York, p. 195, 1980.

Claims

1. Assembly of a first element comprising aluminum nitride and a second element of metal or metal alloy, in which the interface between the two elements comprises at least one simple or complex nitride of rare earth, scandium and / or yttrium.

2. Assembly according to claim 1, in which the first element is made of a material chosen from polycrystalline aluminum nitride, monocrystalline aluminum nitride, a composite metal-aluminum nitride or ceramic-aluminum nitride material comprising at least 40% by volume of aluminum nitride.

3. The assembly of claim 1 or 2 wherein the second element is made of copper or a copper alloy containing at least one precious metal chosen from Ag, Pt, Pd and Au.

4. An assembly according to any one of claims 1 to 3, wherein the rare earth nitride is yttrium or dysprosium nitride.

5. A method of assembling a first element comprising aluminum nitride and a second element of metal or metal alloy, which comprises the following steps: a) placing between the two elements to be assembled a sheet of alloy brazing metallic comprising at least one metal chosen from rare earths, scandium and yttrium, and b) carrying out the brazing of the assembly thus obtained at a temperature of 700 to 1500 ° C. or at a temperature of between 20 and 100 ° C above the melting temperature of the alloy of the solder sheet, optionally in a nitrogen atmosphere.

6. The method of claim 5, wherein the brazing is carried out in a nitrogen atmosphere.

7. The method of claim 5 or 6, wherein the metal alloy of the sheet is a copper alloy comprising at least one metal selected from rare earths, scandium and yttrium.

8. The method of claim 5 or 6, wherein the metal alloy of the sheet is an alloy of copper, at least one precious metal chosen from Ag, Au, Pt and Pd, and at least one metal chosen among the rare earths, scandium and yttrium, possibly containing up to 15% by weight of indium.

9. Method for assembling a first element comprising aluminum nitride to a second metal or metal alloy element, which comprises the following steps: a) depositing on the first element at least one metal or metallic nitride chosen from rare earths, scandium, yttrium and their nitrides, under a nitrogen-based atmosphere, b) subjecting the first element thus coated to a first heat treatment at a temperature of 1000 to 1900 ° C. to react the deposit with the nitride d aluminum, and c) placing above the element thus treated the second element so that it is in contact with the deposit.

10. The method of claim 9, wherein the first heat treatment is carried out in a nitrogen atmosphere.

11. Method for assembling a first element comprising aluminum nitride to a second metal or metal alloy element, which comprises the following steps: a) depositing on the first element at least one metallic nitride chosen from rare earth nitrides, yttrium and scandium, and b) placing the second element to be assembled in contact with the deposit thus formed.

12. Method according to any one of claims 9 to 11, which further comprises an additional step consisting in subjecting the assembly to a heat treatment, at a temperature of 700 to 1500 ° C.

13. Method according to any one of claims 9 to 12, in which the second element is arranged to be assembled on the first element provided with said deposit, by directly forming this second element above said deposit by depositing the metal or the metal alloy intended to form this second element.

14. Method according to any one of claims 5 to 13, wherein the second element is made of copper or a copper alloy and at least one precious metal chosen from Ag, Pt, Pd and Au.