WO2012126955A1

WO2012126955A1 - Thermal interface based on a material having low thermal resistance and manufacturing process

Info

Publication number: WO2012126955A1
Application number: PCT/EP2012/055020
Authority: WO
Inventors: Laurent Divay; Hung LE KHANH; Pierre Le Barny
Original assignee: Thales
Priority date: 2011-03-21
Filing date: 2012-03-21
Publication date: 2012-09-27
Also published as: WO2012126955A4; EP2688974A1; FR2973038A1; FR2973038B1

Abstract

The invention relates to a thermal interface comprising a first layer of aligned nanotubes, characterized in that it additionally comprises a layer of polymer material (C_pol1), said polymer having a carbon-based or siloxane main chain and at least one reactive function, the reactive function being an azide (N₃) that generates covalent bonds with the nanotubes by heating or catalysis and/or the reactive function being a polycyclic aromatic group that generates π interactions between an aromatic group of the polymer and an aromatic group of the first nanotube layer. Another subject of the invention is a process for manufacturing a thermal interface using the preferential adhesion of the nanotubes to the surface of the polymer film relative to that of the nanotubes to the surface of the first growth support.

Description

Thermal interface based on low heat resistance material and method of manufacture

The field of the invention is that of thermal interfaces for electronic components and in particular for power devices.

Usually, these interfaces consist of thermal interface materials (TIM), generally composites (greases, adhesives or gels) consisting of an organic matrix including inorganic conductive thermal particles (metals, nitrides, oxides or carbon). These conventional solutions allow an improvement of the thermal properties of the interfaces. However, the residual strengths remain important and the improvement of their properties is a technical field where the inventive step is important.

More recently, new and more sophisticated solutions have been studied. In particular, the use of vertical nanotubes (VACNT) obtained by vertical growth CVD "Chemical Vapor Deposition", on a substrate is identified as a potentially very powerful solution because of the very high thermal conductivity of the nanotubes in the axial direction (higher at 1000 W / mK). Among the existing solutions, it is possible to note the use of composites comprising vertical nanotubes obtained by CVD growth, on a substrate. But the use of these nanotubes as a thermal interface poses many technical problems. These problems are related to the fact that the vertical nanotubes are grown on a substrate that is not necessarily intended to be part of the final interface. The difficulty is to be able to postpone the nanotubes without their growth substrate in the interface.

One solution is to manufacture a composite material from nanotubes on a substrate as described in US Pat. No. 7,253,442.

In this type of material, the polymer serves as a mechanical support for the nanotubes, so that it is possible to produce a polymer sheet comprising vertical nanotubes. The sheet can then be integrated between the surfaces of the component and the packaging or the heat sink.

Another solution may be the use of a thin substrate on which the nanotubes are raw on both sides. The resulting sheet can then be integrated between the surfaces of the component and the packaging or the heat sink.

Another solution is the integration of the growth substrate in the interface. The obvious solution is the growth of nanotubes on the surfaces of the interface. This solution ensures a good thermal contact nanotube - substrate. The disadvantage is the need to subject these surfaces to the growth conditions of the nanotubes (high temperature, atmosphere and controlled pressures, etc.).

In order not to subject the surfaces to such conditions, one solution is to postpone the nanotubes on one of the two surfaces by exploiting an adhesion. The adhesion on the surface is greater than the adhesion of the nanotubes to their growth substrate, so that the application of a force leads to the separation of the nanotubes from their growth substrate, and the obtaining of an assembly of vertical nanotubes on the new substrate. This adhesion is achieved by the use of a material which may be a metal, a polymer, or simply a chemical function present on the surface of the transfer substrate. It should be noted that the material at the nanotube / transfer substrate interface must have a low thermal resistance since it is an integral part of the thermal interface after transfer.

Metals are the most commonly described materials. Two main modes of implementation can be cited. In a first mode which is similar to a weld, a metal with a low melting point is deposited on the substrate. The end of the nanotubes is brought into contact with the metal and the temperature is raised above the melting point of the metal. The nanotubes are thus welded to the transfer substrate. It should be noted, however, that the nanotubes must be previously treated during the growth step (plasma treatment) so as to allow wetting by the liquid metal and therefore adhesion (Zhu et al., 57th electronic components and technology conference. Reno (Nevada, USA): IEEE, 2007. 2006-10, Zhu et al., Nano Lett., Vol 6, No. 2, 2006). The second method is to evaporate a high-melting (gold) metal on the end of the nanotubes and on the substrate. The contacting of the two metal surfaces and the heating makes it possible to generate an adhesion to the metal-metal interface, even if the temperature is much lower than the melting temperature of the metal. (Johnson et al., Nanotechnology 20 (2009) 065703). Intermediate solutions are described, the use of a gold layer on nanotubes to improve adhesion to a low melting point metal, for example (Tong et al., IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. , NO.1, MARCH 2007).

Organic materials can also be used at the nanotube / transfer substrate interface. The functionalization of the transfer substrate has recently been suggested. The introduction of a reactive surface function with nanotubes (Lin et al., Carbon 48 (2010) 107-1 13) or the use of a functional self-assembled monomolecular layer (Lin et al., J. AM. CHEM. SOC 2008, 130, 9636-9637) make it possible to generate the adhesion of the nanotubes to the transfer substrate. The evaluation of the thermal properties of these interfaces shows relatively low resistances. On the other hand, the transfer substrates are in general silicon layers with very low surface roughness quite far from the applications. The reaction conditions of the molecular functions of the nanotubes are, moreover, very specific (working under nitrogen, using microwave irradiations). However, it is demonstrated that the existence of a strong (covalent) interaction between the nanotube and the transfer substrate is favorable to a low thermal resistance. The effect of the covalent interaction has also been demonstrated by calculation (Hu et al., Journal of Applied Physics 104, 083503 (2008)).

A last solution consists in the use of polymers. The polymers have an excellent affinity for the nanotubes, so that obtaining adhesion is easy. The polymers are therefore already used for the transfer of nanotubes (Tsai et al., APL 95, 013107 (2009)). However, because of their low thermal conductivity, they are generally not used in thermal interfaces.

Their use for these applications seems however possible, provided that the thickness of the polymer is very low so that the associated thermal resistance is low. Furthermore, the need to perform this transfer at a relatively low temperature requires that the polymer has a low glass transition temperature. These two points do not a priori allow sufficient adhesion for the transfer of nanotubes, which greatly reduces their applicability. This is why, and in this context, the present invention proposes a solution allowing the use of polymers at the interface: nanotubes of the CNT carbon nanotube type / transfer substrate, which make it possible, on the one hand, to transfer the CNTs to the CNT. new substrate, and on the other hand the obtaining of very low thermal resistances. The formulation of the proposed polymers makes it possible to obtain a strong adhesion with the nanotubes even if their glass transition temperature Tg is very low, and if they are used in very thin layers.

More specifically, the present invention relates to a thermal interface comprising a first layer of aligned nanotubes which further comprises a polymer layer (C _po n) in contact with said first layer, said polymer having a carbon or siloxane main chain and at least a reactive function, the reactive function being an azide (N ₃ ) generating covalent bonds with the nanotubes by heating or catalysis and / or the reactive functional group being a polycyclic aromatic group generating a π-type interaction between an aromatic group of the polymer and a aromatic group of the first nanotube layer.

An π type interaction between two aromatic groups is commonly called "ττ-stacking".

According to a variant of the invention, the interface further comprises a second polymer layer having a carbon or siloxane main chain and at least one reactive functional group which may be an N ₃ azide or an N ₂ ⁺ diazonium or an NH group. ₂ or Si-H, and generating covalent bonds with the nanotubes, the nanotube layer being inserted between said polymer layers.

The covalent bonds can be done either with the aromatic rings of the carbon nanotubes, or with the defects present on the nanotubes.

The ττ-stacking interactions are done with the aromatic rings of the nanotubes.

According to a variant of the invention, said polymer comprises a first series of side chains carrying the reactive function and a second series of nonfunctional side chains. According to a variant of the invention, said polymer has side chains and corresponds to the following chemical formula:

According to a variant of the invention, said polymer corresponds to the following chemical formula, namely:

The subject of the invention is also a method for manufacturing a thermal interface according to the invention, characterized in that it comprises the following steps:

the production of a layer of nanotubes on the surface of a first growth support;

the production of a polymer layer on the surface of a second support, said polymer being a polymer having a carbon or siloxane main chain and at least one reactive function, the reactive function being an N ₃ azide generating covalent bonds with the nanotubes and / or the function being a polycyclic aromatic group generating an interaction between an aromatic group of the polymer and an aromatic group of the first nanotube layer.

- the assembly of the two supports;

removing the first support from the preconstituted assembly, the adhesion of the nanotubes to the polymer film surface being preferential to that of the nanotubes on the surface of the first growth support.

According to the invention, the assembly of the two supports is carried out under pressure.

According to a variant of the invention, the assembly of the two supports is carried out at a temperature below 80 ° C.

The subject of the invention is also a method of manufacturing a device characterized in that it comprises the steps of the method of manufacturing a thermal interface according to the invention and which can comprise, after the withdrawal of said first support:

the direct report of a third support;

the production of a composite, in particular by the infiltration of an organic matrix into the nanotubes, followed by the transfer of a third support;

depositing a metal layer to bond the nanotubes to a third support;

the deposition of self-assembled monomolecular layers on a third support;

depositing any other type of polymer made in a thin layer on a third support.

The subject of the invention is also a device comprising a first support and a third support that can in particular be a housing, a heat sink, characterized in that it further comprises a thermal interface according to the invention, said interface being located between the two supports.

The device can be an electronic device, one of the supports comprising at least one electronic component, it can also in particular, be a device incorporating a laser, a thermal interface of the invention and a heat sink.

The invention will be better understood and other advantages will become apparent on reading the description which follows given by way of non-limiting example and by virtue of the appended figures among which:

FIGS. 1a to 1d illustrate the various steps of a method of manufacturing a thermal interface according to the invention;

FIGS. 2a and 2b illustrate the various steps of a method of manufacturing an electronic device comprising the interface of the invention.

In general, the qualities of the proposed polymers to ensure a good interface can be summarized in two main points:

the presence of a covalent interaction with the CNTs and / or the presence of an ττ-stacking interaction;

a very low glass transition temperature Tg.

These cumulative properties allow several possibilities in the context of the intended application:

- the easy transfer due to the stronger adhesion than the adhesion of VACNT on their growth substrate;

the covalent interaction makes it possible to retain the adhesion even if the polymer has a very low Tg;

the covalent interaction makes it possible to retain the adhesion even if the polymer is disposed in the form of a very thin layer (which is favorable for limiting the thermal resistance);

a very low glass transition temperature Tg allows a good wetting of the CNTs even at very low temperature;

optimization of the VACNT / polymer interface thermal resistance thanks to the covalent interaction;

- a possible transfer to different types of substrates because of the nonspecific polymer / substrate interaction (the use is, however, restricted to substrates that allow wetting by the polymer). Accordingly, the advantages of this technique for the application of the present invention are: the formation of an interface by simple heating at low temperature;

- the formation of an interface without vacuum technique;

- the formation of a low temperature interface compared to metals;

an adhesive interface;

- excellent thermal properties (very low resistance)

The Applicant also proposes polymer formulations that can generate covalent bonds and / or ττ-stacking bonds.

These formulations respond to molecules such as copolymers

The flexibility of the macromolecular skeleton allows polycyclic aromatic groups to come close to the carbon nanotubes and thus generate Π stacking.

The grafting of a copolymer via the azide function is carried out by heating, typically to a temperature of 135 ° C. for 24 hours. The ττ-stacking bond is spontaneously at room temperature.

The Applicant has tested functional azide side chain polymer formulations. It is well known that the azide functions are reactive on the walls of the nanotubes. Moreover, the length lateral chains is adapted to obtain a glass transition temperature Tg, low.

These formulations correspond to the following general formulation

The general formulation of the polymer is known. The reactive function can be an azide (N ₃ ) or a diazonium (N ₂ ⁺ ) or another type of reactive function

(NH ₂ , Si-H, etc.)

The polymers tested are derivatives of polyvinylphenol (main chain). The phenol function is used for the introduction of different reactive grafts (carrying azides), or nonreactive alkyl chains to adjust the glass transition temperature.

More specifically, the following polymers have been tested:

The polymer P1 having a glass transition temperature Tg =

1 10 ° C

The polymer P2 having a glass transition temperature Tg = 60 ° C.

The polymer P3 having a glass transition temperature Tg

The grafting reactions having a yield less than 100%, phenol functions remain in the final polymer (a few%).

By way of example, the synthesis of the polymer P3 is described below:

Synthesis of DPTS (4-dimethylamino pyridinium paratoluene sulphonate)

In a 250mL Erlenmeyer flask with magnetic stirring, the DMAP (4-dimethylaminopyridine) is dissolved in 125 ml of ether. In parallel, the APTS.H ₂ 0 (para-toluenesulphonic acid monohydrate) is dissolved in a mixture of 6 ml of methanol and 10 ml of ether. This solution is added dropwise into the DMAP solution. The salt precipitates. Stir at room temperature for 1 hour.

The salt is filtered off from the fritted glass, then washed with ether and dried under vacuum to give 4 g of product in a yield of 91.3%.

Synthesis of the graft (6-azido hexanoic acid)

In a three-necked flask of 250 ml, 3.9 g of 6-bromohexanoic acid (20 mmol) are dissolved in 100 ml DMF and then 3.9 g NaN ₃ (60 mmol) are introduced. The reaction mixture is heated overnight at 85 ° C and then cooled. DMF is evaporated. The products are redissolved in 100mL dichlomethane (CH ₂ Cl ₂ ). The organic phase is washed with water acidified with HCl (50 mL HCl 25% + 750 mL H ₂ O) and then dried with Na ₂ SO ₄ . The solvent is then evaporated under reduced pressure. 2.8 g of 6-azidohexanoic acid is obtained with a yield of 89%.

Grafting on the polymer:

In a three-necked flask of 100 ml (THF) are introduced 120 mg of poly (4-vinylphenol) (Mw = 25,000), 320 mg of 6-azido hexanoic acid, 494 mg of DCCI (Ν, Ν'-Dicyclohexylcarbodiimide) and 164 mg of DPTS ( paratoluene 4-dimethylamino pyridinium sulfonate). The solution is stirred for 24 hours at room temperature under a stream of argon.

After the reaction, the urea is removed by filtration. Urea is washed with CH ₂ Cl ₂ to recover the polymer. The filtrate is concentrated under reduced pressure. The product is precipitated in methanol. 50 mg is obtained after drying under vacuum. The yield is 20%.

Example of setting a polymer according to the invention to use:

The different steps of an exemplary method of implementation are illustrated by means of FIGS. 1a to 1d.

In a first step illustrated in Figure 1a, is deposited by spin-coating the polymer on a copper substrate from a solution containing the polymer _p0 s i _y i - This gives a thin film of about 30 nm of polymer C _po n on a support Si copper, after possible annealing.

In parallel, vertical nanotubes (VACNT) of 10μιη are used on a silicon growth substrate S ₀ and the nanotubes are brought into contact with the silicon growth substrate as illustrated in FIG. 2b, constituting a layer of nanotubes CM-

A pressure of about 66 kPa is applied at about 80 ° C for about 90 minutes to effect the assembly illustrated in Figure 1c. Thermal measurements are then made.

Finally, verifications are made on the possibility of transfer as illustrated in FIG. 1 d, corresponding to the opening of the assembly, releasing the support S ₀ .

The applicant has carried out test measurements to evaluate different types of thermal and mechanical properties.

Thermal properties:

The thermal properties of the CNT / polymer / copper contact are tested by measuring the thermal resistance of the assembly. The resistance is compared to the resistances of other assemblies made in different ways.

Mechanical properties :

The possibility of nanotubes being postponed by this technique is simply tested by verifying the possibility of transferring the nanotubes to the copper substrate.

The assembly is opened by mechanical traction on the copper. It is found that the nanotubes remained on the copper. Structural characterizations:

The nanotubes reported on the copper are observed under a scanning electron microscope.

It is found that the nanotubes remained vertical and parallel. First example of manufacture of a device according to the invention integrating an interface of the invention:

According to this example, the polymer layer is formed on the heat sink, the nanotubes are thus transferred to it as explained above, and the dissipator-component assembly is made directly without prior treatment.

Second embodiment of a device of the invention

The electronic device comprises a printed circuit type support and comprising on the surface at least one component Comp, which can be a power component, generating a large amount of heat in operation.

A polymer layer C _p0 i _y 2 of the same type as the polymer used in the thermal interface of the present invention is deposited on the surface of said component, as illustrated in FIG. 2a.

The electronic component covered with the layer C _p0 i _y 2 is then assembled with the preliminary interface prepared and comprising the stack of a heat-sink type support Int, of the polymer layer called the first layer C _p0. i _y i and CM nanotube layer.

An electronic device incorporating a quality thermal interface as shown in FIG. 2b is thus obtained.

Claims

1. Thermal interface comprising a first layer of aligned nanotubes characterized in that it further comprises a polymer layer (C _po n) in contact with said first layer, said polymer having a carbon or siloxane main chain and at least one reactive function, the reactive function being an azide (N ₃ ) generating covalent bonds with the nanotubes by heating or catalysis and / or the reactive functional group being a polycyclic aromatic group generating π-type interactions between an aromatic group of the polymer and an aromatic group of the first layer of nanotube.

2. Thermal interface according to claim 1, characterized in that it further comprises a second layer (C _p0 i2) of polymer having a carbon or siloxane main chain and at least one reactive function, the reactive function being an azide (N ₃ ) or a diazonium (N ₂ ⁺ ) or an NH ₂ or Si-H group, and generating covalent bonds with the nanotubes and / or the reactive functional group being a polycyclic aromatic group generating π-type interactions between an aromatic group of the polymer and an aromatic group of the first nanotube layer, the nanotube layer being inserted between said polymer layers.

3. Thermal interface according to one of claims 1 or 2, characterized in that said polymer comprises a first series of side chains bearing the reactive function and a second series of nonfunctional side chains.

4. Thermal interface according to one of claims 1 or 2, characterized in that said polymer is a side chain polymer and has the following chemical formula:

5. Thermal interface according to one of claims 1 or 2, characterized in that said polymer is a side chain polymer and has the following chemical formula:

6. Thermal interface according to one of claims 1 or 2, characterized in that said polymer is a copolymer meets the following chemical formula:

7. Thermal interface according to one of claims 1 or 2, characterized in that said polymer is a copolymer corresponding to the following chemical formula:

8. Electronic device comprising two supports, one of which may be heat sink type or housing, characterized in that it further comprises a thermal interface according to one of claims 1 to 6, said interface being located between the two supports.

9. A method of manufacturing a thermal interface according to one of claims 1 to 6, characterized in that it comprises the following steps: the production of a layer of nanotubes (CN on the surface of a first growth support (S ₀ );

- Making a polymer layer (C _po n) on the surface of a second support (l _n t), said polymer having a carbon or siloxane main chain and at least one reactive function, the reactive function being an azide (N ₃ ) and generating covalent bonds with the nanotubes, and / or the reactive functional group being a polycyclic aromatic group generating π-type interactions between the aromatic group of the polymer and an aromatic group of the first nanotube layer;

- the assembly of the two supports;

10. A method of manufacturing a thermal interface according to claim 8, characterized in that the assembly of the two supports is carried out under pressure.

1 1. A method of manufacturing a thermal interface according to claim 9, characterized in that the assembly of the two supports is carried out at a temperature below 80 ° C.

12. A method of manufacturing a device characterized in that it comprises:

the steps of the method of manufacturing a thermal interface according to one of claims 8 to 10;

after the withdrawal of said first support, the direct transfer of a third support (S).

13. A method of manufacturing a device characterized in that it comprises:

the steps of the method for manufacturing a thermal interface according to one of claims 8 to 10, after removal of said first support, the production of a composite, in particular by the infiltration of an organic matrix into the nanotubes, followed by the transfer of a third support.

14. A method of manufacturing a device characterized in that it comprises:

after the withdrawal of said first support, the deposition of a metal layer to weld the nanotubes onto a third support.

15. A method of manufacturing a device characterized in that it comprises:

after removal of said first support, the deposition of self-assembled monomolecular layers on a third support.