JP2023060560A - Method for joining highly crystalline carbon substrate and laminate - Google Patents

Abstract

To provide a laminate joined body in which a multilayer film is formed of a graphene layer by a gentle bond by a van der Waals force in a thickness direction of a crystalline carbon substrate, is likely to be peeled and scattered by a comparatively small external force, and thermal conductivity is sacrificed by an organic adhesive in joining the substrates.SOLUTION: The present invention provides a crystalline carbon substrate with excellent flatness by covering a detailed part of a substrate with a metal atom by a vacuum film-formation process, forming an anchor effect, and further a carbide bond and an oxycarbide bond to form a strong bond, and with Sn, In or a low melting point metal film such as an alloy film is used as an uppermost layer, performing pressurization by a press surface mirror-finished at a temperature near a melting point or a temperature equal to or higher than the melting point, and a laminate in which thermal conductivity in a thickness direction is not impaired by joining the metals by atomic diffusion.SELECTED DRAWING: Figure 8

Description

The present invention relates to a crystalline carbon substrate material such as graphite or graphene, which is used as a heat dissipation/cooling mechanism for semiconductors, electronic devices, and in-vehicle devices. In addition, by forming a low melting point metal film on the outermost surface layer and atomic diffusion bonding between the metals, the thickness can be reduced while maintaining high thermal conductivity. Kind Code: A1 Methods and laminates for producing thick laminates are provided.

Due to the increasing speed of CPU circuits and other circuits, the increasing brightness of display devices, the high-density electronics of in-vehicle devices, and the electrification of automobiles, these devices tend to generate heat. There is an increasing possibility that the temperature of the equipment concerned will increase, and furthermore, failures will be induced due to the high temperature part becoming a spot state, resulting in a lack of reliability. As one of the countermeasures, it is becoming more and more important to quickly diffuse and homogenize the heat generation state, and furthermore, to dissipate the heat to the outside such as the atmosphere.
As a means, conventional heat sink structures made of metals such as aluminum and copper heat sinks, grease-like substances kneaded with particles of metals and compounds with good thermal conductivity, and sheets mixed with silicon polymers. As a result, the heat spot is diffused, homogenized and heat is dissipated.

One of the important indices for the heat diffusion function is the thermal conductivity (W/m・K). For example, silver is 418, copper is 398, aluminum is 237, and copper and its alloys are used as heat sinks and heat pipes. is often used. However, metals have a high specific gravity, for example copper has a specific gravity of 8.96, and in various devices that require even a slight reduction in weight, polymer sheets kneaded with highly thermally conductive particles are used. This also reduces the specific gravity, and the shape and thickness can be relatively freely adjusted according to the place of use. The problem is that, although it depends on the ratio of kneading with heat-conducting particles, in general, even if graphite particles are kneaded, only a low thermal conductivity of about 10 W/m·K can be obtained.

By using a substrate of a highly crystalline carbon compound typified by carbon graphite as a thermally conductive material, a thermal conductivity exceeding 1,500 W/m·K in the crystal direction can be obtained. In order to obtain a graphite substrate with high thermal conductivity, for example, a method of carbonizing a polyimide film at an ultra-high temperature of 3,000°C or a method of growing graphene crystals for a long time by CVD (chemical vapor deposition) is used. There is a way.

In the planar direction (x-y) of graphene, carbon (C) forms a covalent bond and is strongly connected, but in the thickness direction (z), the graphene layers are stacked, and the bonding between the layers is is only due to weak van der Waals forces. Therefore, both the interlayer and the graphene single layer are extremely weak against external impacts, and may easily peel off and scatter.

発熱体からの熱伝導を効率よく行うためには，発熱体との接触面積を増やさなければならないが，凹凸部分との接触が空気層となりやすいため，熱伝導量を低下させることになる。そのため，基板表面の平坦化は熱伝導量を向上させる有効な手段である。 In order to increase the strength between the graphene layers, the carbonization process is devised, and the bulk strength of the graphite sheet is ensured by bending the laminated graphene body (Fig. 1). , it is difficult to say that the strength has improved from a microscopic point of view. In addition, due to such processes and structures, the sheet surface has many irregularities, and the surface roughness exceeds 10 μm in terms of Rz.
As an example, Table 1 shows the results of surface roughness measurement using a laser microscope.
table 1

In order to conduct heat efficiently from the heating element, the contact area with the heating element must be increased. Therefore, flattening the substrate surface is an effective means of improving the amount of heat conduction.

Furthermore, as mentioned above, the process of producing crystalline graphite involves the dehydrogenation of an organic polymer film such as polyimide in an ultra-high temperature environment. , so-called dangling bonds, which are highly reactive. This was confirmed by the electron spin resonance and the formation of CS bonds through adsorption reactions with triazine-based compounds.
Graphene particles are thus highly active, and as described in the prior art literature, attention has been drawn to the effects of such particles on the respiratory system.

Attempts to coat the surface of crystalline graphite sheets with metal have been conducted as one of the studies of lamination bonding technology, using metals such as Ti, Zr, and Al that react with carbon to form carbide bonds, and catalysts. Patent Document 1, Patent Document 2, etc. describe a structure in which the layers are laminated and bonded by reacting them at a high temperature. However, all of them require a high treatment temperature of about 800 to 1000°C.

JP 2015-532531 (Registration 6529433) JP 2020-181926

National Institute of Advanced Industrial Science and Technology Handling of exfoliated graphene (2017)

In order to make a crystalline carbon substrate with high thermal conductivity by dehydrogenation reaction at an ultra-high temperature, there is a trade-off relationship between the thickness and thermal conductivity of the completed substrate, considering the ease with which hydrogen can escape from the inside of the substrate. Therefore, there is a method of making a thin substrate and laminating it with an organic adhesive, but the thermal conductivity of the organic adhesive is low, so the thermal properties in the thickness direction are greatly impaired.
Metals such as Ti and catalysts can be used to improve handleability and can be bonded to metal plates by creating carbide bonds (e.g., C-Ti) as described in prior patent documents, but at 800 ° C High temperature treatment at ~1,000°C is required.
In addition, since the bond in the thickness direction is based on the van der Waals force, it is weak against external forces and relatively easily peels off and scatters from the substrate.

In the present invention, the surface of the substrate is metallized at an extremely low substrate temperature, such as room temperature, by a vacuum deposition process, thereby forming an anchor effect and carbide bonds, etc., and handling is improved, and the crystal is free from peeling and scattering. provide a flexible carbon substrate.
Furthermore, by making the outermost surface of the metallized substrate a low-melting-point metal, it is possible to bond multiple substrates at an overwhelmingly lower temperature than before, such as around 100°C to 300°C. Methods and laminates for co-bonding are provided.

As described above, the crystalline carbon substrate has multiple layers of graphene crystals bent within the substrate sheet to improve bulk strength. Its cross section is shown in Fig.1. As for the surface roughness, as shown in Table 1, there is a problem of flatness when joining multiple sheets. Figure 2 shows the composition in the depth direction of the carbon substrate measured by X-ray photoelectron spectroscopy (XPS analysis). The Ar2p spectrum was observed in the spectrum after the substrate surface was etched with an Ar ion beam. It can be seen that there is a space so small that Ar atoms can be trapped in the carbon substrate.

Even if the substrate surface having such characteristics and the metal plate are reacted with a catalyst at a high temperature, it is difficult to react in the details of the fine recesses of the carbon substrate. However, in the present invention, when vacuum film formation is used, the metal atoms enter even minute portions, so that the gaps that reduce the thermal conductivity can be reduced, and at the same time, the anchor effect can be used, and the substrate and the metal film can be separated. Adhesion is improved. Furthermore, XPS analysis revealed that carbon atoms and metal atoms are activated by plasma during deposition, forming carbide bonds and oxycarbide bonds. It was found that such bonds can be formed at temperatures around room temperature.

The metal film in the present invention is Ti, Si, Ni, Al, Fe, Cu, Sn, In, and alloys thereof, and Ti, Si, etc. can form carbide bonds to ensure adhesion. Sufficient adhesion could be obtained due to the anchor effect even with metals that are difficult to form. As a result, the strength of the substrate sheet can be ensured to improve handleability, and furthermore, the carbon substrate can be prevented from peeling off and scattering due to an external force, thereby greatly improving safety.

In the present invention, the metal film on the outermost surface can be made of a low melting point metal such as Sn or In or an alloy, and depending on the application, it is also possible to have a structure in which a high melting point metal such as Ti or Si is deposited as a lower layer. It is possible. In the case of a low-melting-point metal, it can be flattened by a pressing device, and the pressing temperature is preferably near the melting point. In addition, by pressing at a temperature above the melting point and releasing the pressure at a temperature below the melting point, it is possible to flatten with a low pressing pressure.In this case, the temperature is lowered to a temperature lower than the melting point and the pressure is released. By doing so, the flatness of the press surface can be transferred. This flatness can increase the contact area with heat sources and heat sinks, and can maximize the amount of heat transfer when they interatomically diffuse with low-melting-point metals.

Furthermore, in the present invention, low-melting-point metal films, such as Sn films, deposited on the outermost surface of the crystalline carbon substrate can be brought into contact with each other and bonded by diffusing atoms. As a result, a plurality of carbon substrates could be easily laminated and bonded together.
In general, the surface oxide film of Sn foils produced by rolling or the like exists relatively deep, and the XPS analysis shows that it is approximately 20 nm or more, so it was necessary to reduce the surface oxide film in order to join the foils together. However, the oxidized layer on the surface of the vacuum-deposited Sn film is less than 1 nm even after a considerable amount of time has passed after the deposition, and it was found that the stack can be easily bonded by pressing the stack at a temperature above its melting point. rice field.

In the present invention, the surface of a carbon substrate is metallized by a vacuum film-forming process. By performing vacuum film-forming after forming holes in the substrate in advance, metal can be formed on the inner walls of the holes. As a result, not only the anchoring effect of the metal film from the surface, but also the bonding by the van der Waals force can be significantly strengthened by covering the inner walls of the holes with the metal film.
Although the diameter of the holes ranges from about 50 μm to about 500 μm, the present invention is not influenced by the size of the diameter, and the density of hole processing can be determined appropriately according to the situation. The hole processing method may be laser processing or mechanical processing such as a drill or punch.

Electron microscope (SEM) cross-sectional photograph of a crystalline carbon substrate (10,000x) Electron microscope (SEM) surface photograph of crystalline carbon substrate (300x) XPS spectrum of the surface of the crystalline carbon substrate Top layer of Ti film on crystalline carbon substrate and C1s spectrum at a depth of about 10 nm SEM photograph of a Sn film (500 nm) formed on a crystalline carbon substrate (300x magnification) Optical micrograph of the surface of a crystalline carbon substrate with holes processed by a laser processing machine Analysis of film formation state of metal atoms in hole processed parts by vacuum film formation Schematic diagram of lamination bonding of crystalline carbon substrates XPS analysis result of oxide film on crystalline carbon substrate with Sn500nm film SEM photograph of bonded carbon substrate surface (100x magnification)

Figure 1 is a SEM photograph (10,000x magnification) of a cross section of a typical crystalline carbon substrate. The graphene layers are stacked in a bent state. As a result, flatness is lost, but strength as a bulk is ensured.

Figure 2 is a SEM photograph (3,000x magnification) of a typical crystalline carbon substrate surface, showing unevenness and steps. In addition, there is a part that is about to peel off at the cut portion of the step, and such a part can be fixed by the metallization according to the present invention.

FIG. 3 shows an XPS spectrum of a depth of about 5 nm obtained by etching the surface of a crystalline carbon substrate with an Ar ion beam of 2 kV for 0.5 minutes. An Ar2p spectrum is confirmed near 242 eV, indicating that Ar atoms are trapped in the carbon substrate and that there are very fine cavities.

Ti is known to be a metal that has a very high affinity with graphite, but in general, it is reacted at a temperature of 800°C or higher using a catalyst to form a Ti-C carbide bond for bonding. Figure 4 shows an example of XPS analysis of the surface of a crystalline carbon substrate on which a Ti film was formed by sputtering at a substrate temperature of 50°C to a thickness of 200 nm. The top layer of the Ti film has a C1s spectrum near 284.8 eV, which is derived from general organic deposits. A C-Ti carbide bond with a peak is formed, and this bond due to C-Ti is observed up to the boundary with the 200 nm carbon substrate where the Ti film disappears.

Figure 5 is an SEM image of a vacuum film of approximately 500 nm thick Sn metal deposited directly on a crystalline carbon substrate at a substrate temperature of 50°C. Compared with FIG. 2, the fine flatness is greatly improved, and it can be seen that the detachment of the graphite particles on the surface can be prevented, and the substrate can be prevented from peeling off and scattering due to a slight external force.
In order to further improve the flatness, in the present invention, pressing near the melting point of Sn, or at a temperature slightly above the melting point, for example, about 235 ° C, and then lowering the temperature below the melting point and releasing the press pressure. The specular surface of the press platen can be transferred to the Sn film surface.

FIG. 6 is an optical microscope photograph of a hole diameter of 200 μm processed by a picosecond laser with a wavelength of 532 nm. As an example, a 400 nm film of Al was sputtered on this substrate, and the composition analysis of its cross section is the result of SEM-EDS. As shown in FIG. 7, an Al atomic concentration of 50 at % or more is confirmed up to a depth of about 25 μm, and most of the wall surface is covered with Al metal. Therefore, by performing vacuum deposition from both sides, the entire wall surface can be metalized, and the graphene layer, which is composed of a weak van der Waals force, can be strengthened even in the depth direction.

The holes in the present invention are capable of absorbing the excess amount of the metal, such as Sn, which is in excess of the pressing pressure when the low melting point metal such as Sn is pressed at a temperature close to or above the melting point, thereby further strengthening the substrate. Become.

Generally, the thermal conductivity in the thickness direction of a crystalline carbon substrate is about 5 W/m/K, but the thermal conductivity of the metal used in the present invention (Table 2) is 10 to 50 times that of the substrate. As described above, by increasing the filling rate of the holes, it is possible not only to prevent the decrease in thermal conductivity in the thickness direction due to the holes, but also to increase it.
It is assumed that the hole area S1 and the conduction amount S1×η1 lost due to the thermal conductivity η1 of the substrate are filled in the area S2 with the material having the thermal conductivity η2. When η1 = 5 (W/m/K) and S1 = 100, assuming that the thermal conductivity of the filling material η2 = 100 W/m/K, S2 = 5 is equivalent. That is, in this case, if the filling area is 5% or more, it is possible to improve the thermal conductivity 5 W/m·K in the thickness direction of the substrate.
Table-2

In general, the surface of a crystalline carbon substrate has highly active dangling bonds, but it is difficult to directly bond the surfaces of the substrate, and the production of a thick substrate sacrifices thermal conductivity. Alternatively, the chemical vapor deposition (CVD) process can be used to react for a long period of time.
In the present invention, the surface of a crystalline carbon substrate is metallized, in particular, a low melting point metal or the like is formed on the uppermost layer by a vacuum film formation process, and the metal surface is pressed at a temperature near or above the melting point to bond the metals together. After mutually diffusing, the pressure is released at a temperature below the melting point, whereby a thick bonded body of crystalline carbon substrates can be produced.

FIG. 8 is a schematic diagram of the apparatus. A plurality of crystalline carbon substrates 1 with a low-melting-point alloy film 2 formed on the uppermost layer are stacked and set between a stage 3 that can be heated and pressurized in a chamber 4. Apply pressure according to the temperature profile.
It is generally known that metal foils produced by rolling, such as Sn foils, have an oxide film on the surface up to about 200 nm. However, as shown in Fig. 9, the Sn film formed in a vacuum according to the present invention has only a thickness of about 1 to 2 nm on the surface, and is easily oxidized when mechanical pressure is applied at a temperature near or above the melting point. The film is destroyed, and Sn atoms are mutually diffused and bonded. As a result, it was found that the bonding can be achieved without reduction treatment or treatment in a reducing atmosphere, but this does not deny the reduction treatment or the atmosphere.

Although the present invention uses a low-melting-point alloy for the uppermost layer for planarization and bonding, other low-melting-point metals can be used in addition to Sn and In. Table 3 shows some of the Sn-based metals specified in JIS Z3282 (2017).
Table-3

Although the present invention will be explained with the following materials and manufacturing methods, the present invention is not limited by these.

As a crystalline carbon substrate, a graphite sheet GD-040 film (t=40 μm) (Gurdnec) was cut into 40 mm×60 mm, ultrasonically cleaned in isopropanol for 5 minutes, and dried. Next, the film was set in a sputtering deposition system (MPS-3000: ULVAC), and a Sn target (Toshima Seisakusho Co., Ltd.) was used to form a Sn film with a thickness of 500 nm at a substrate temperature of 50°C in an Ar atmosphere. .

A 20 mm × 20 mm piece was cut from the film-formed substrate, sandwiched between mirror-finished quartz glass substrates (50 mm × 50 mm × 2 mm), and pressed with a heating and pressure press equipped with a temperature controller manufactured by Miclab Co., Ltd. It was pressed at a temperature of 235°C at 5 MPa for 5 minutes, cooled to 220°C, released the pressure, and taken out.

The surface roughness of the substrate was measured by Lasertec's OPTELICS Hybrid+, and Ra < 1 μm, Rz ≒ 1 μm, and it was confirmed that the surface roughness of the crystalline carbon substrate shown in Table 1 was greatly improved. Further, it was confirmed with an optical microscope that carbon powder did not occur even when the metal surface of the substrate was bent.

A piece of 20 mm×20 mm was cut out from the substrate prepared in Example 1, sandwiched between 100×100 mm SUS substrates facing each other, and pressed with a heating press with a temperature controller manufactured by Miclab Co., Ltd. It was pressed at 5 MPa for 2 minutes at a temperature of 235°C, cooled to 220°C, released the pressure, and taken out.

An X-ray CT (Shimadzu SMX-90CT) was used to confirm that there were no voids on the bonded surface of the bonded sample. Furthermore, the substrates were peeled off with tweezers and the surface was observed. Its SEM photograph is shown in FIG. The white part is the Sn foil, and the black part is the carbon substrate.
Although this experiment was carried out at atmospheric pressure, it is expected that the generation of minute voids can be suppressed by heating and pressing in a vacuum.

The present invention improves safety and handling by surface metallization, maintains and further improves the thermal conductivity in the thickness direction, and achieves a thick crystal while maintaining a high thermal conductivity in the planar conduction direction. provide a flexible carbon substrate.

REFERENCE SIGNS LIST 1 crystalline carbon substrate 2 low-melting-point metal thin film 3 temperature-adjustable pressure stage 4 vacuum chamber

Claims (4)

A crystalline carbon substrate in which a single-layer or multiple-layer metal film with a thickness of 100 nm or more is formed on the substrate surface by a vacuum deposition process, and the surface is flattened by heating and pressing the entire surface near the melting point of the metal film. and a laminate obtained by laminating and joining the metal surfaces The metal is at least one metal or alloy of Ti, Si, Ni, Al, Fe, Cu, Sn, and In, and the melting point of the top layer metal is 300 ° C. or less. Crystalline carbon substrates and laminates described After forming a plurality of through-holes with a diameter of 20 μm or more in the substrate, a metal film is formed by a vacuum film-forming process, and the metal is formed on the inner wall of the through-holes at the same time as the substrate surface. 3. The crystalline carbon substrate and laminate according to claim 1 and claim 2, wherein the metal or alloy material thereof is made to penetrate into the through-holes by hot pressing. The flattening of the crystalline carbon substrate surface and lamination bonding according to any one of claims 1 to 3, characterized in that the heat pressing is performed at a temperature above the melting point of the uppermost layer metal, and the pressure is released after the temperature is lowered below the melting point. The crystalline carbon substrate and laminate according to claims 1 to 3, characterized by

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