JP2010114120A - Heat dissipating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat dissipating method for transmitting heat from a heat generator to a heat sink, capable of excellently releasing a thermal stress and radiating heat, irrespective of the difference in thermal expansion coefficient between the heat generator and the heat sink. <P>SOLUTION: The heat dissipating method includes transmitting heat from the heat generator to the heat sink, the difference in thermal expansion coefficient between them being ≥0.5×10<SP>-6</SP>/K, by allowing the heat generator and the heat sink to contact each other. In this method, the heat sink has a substrate in which at least part of a surface contacting the heat generator is SiC and a BN nano-tube layer formed on the whole or part of the surface or both surfaces of the substrate, and allows the BN nano-tube layer formed on the substrate surface of the heat sink to contact the heat generator. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a heat dissipation method that exhibits high heat dissipation while reducing thermal stress by contacting with heat generating elements having different thermal expansion coefficients or inserting them between the heat generating elements and the heat dissipating elements.

  As personal computers and mobile electronic devices become more sophisticated and densely mounted, the amount of heat generated per unit area of heat sources such as CPUs, GPUs, chipsets, and memory chips has increased dramatically. High performance is required. This is because the semiconductor element has an operating upper limit temperature specific to the material constituting the semiconductor element, and the element is destroyed at a temperature higher than that temperature, so that the life is significantly reduced in a state where heat radiation is insufficient. Usually, natural convection or forced convection using an electric blower is used to radiate heat, but in principle there is an upper limit specific to the cooling method for the amount of heat radiated per unit area, so to dissipate a large amount of heat, In general, a heat dissipating device called a heat sink or a heat spreader that expands a heat dissipating area is used.

Specifically, it is several to several heat dissipation surfaces of a semiconductor element (hereinafter referred to as a die) (generally, a semiconductor element forms a functional part made of a thin film on one surface of a Si single crystal substrate and dissipates heat from the opposite surface). Heat generated by contact with a heat sink material made of metal (copper or aluminum is generally used) having a surface area ten times higher and high thermal conductivity is transferred from the die to the heat sink.
At this time, there is a large difference in the coefficient of thermal expansion between the semiconductor element and the metal heat sink, so if they are stacked as they are, thermal stress is generated at the interface between the two under a temperature cycle, and the semiconductor element is distorted to stabilize the device. It may not work, or in the worst case, it may lead to cracking or peeling at the interface or destruction of the semiconductor element.

In order to deal with such problems, a means for reducing thermal stress by inserting an intermediate heat dissipation material called a heat spreader between a semiconductor element and a heat sink is often used. As the heat spreader material, a composite material such as CuW, CuMo, or Al-SiC is used (see, for example, Patent Document 1). The thermal expansion coefficient of these composite materials is determined by the volume content of components constituting the composite materials. Therefore, the composition is adjusted and manufactured so as to have a moderate value between the thermal expansion coefficients of the semiconductor element and the heat sink. These heat spreaders are joined to a semiconductor element or a heat sink through brazing or Au—Sn plating.
However, conversely, it is necessary to change the composition of these composite heat spreaders depending on the combination of the semiconductor element and the heat sink to be used, and it is necessary to prepare products with various compositions in advance. There was a problem that the manufacturing cost of the composite material itself was expensive.

JP 2006-108317 A Japanese Patent No. 3183845 JP 2000-109306 A

  Therefore, in order to solve the above-described problems, the present invention provides a heat dissipation method for transferring heat from a heating element to a heat dissipation body, which can relieve thermal stress well regardless of the difference in thermal expansion coefficient between the heating element and the heat dissipation element. It is an object of the present invention to provide a heat dissipation method that can dissipate heat.

As a result of intensive investigations to solve the above problems, the inventors have used any heat spreader in which a stress relaxation layer made of carbon nanotubes is formed on the surface of a substrate having high thermal conductivity. It was found that the thermal stress can be effectively relieved even for the combination. That is, this invention relates to a heat dissipating method for transferring heat from a heat generating element to a heat dissipating element by contacting the heat generating element having a difference in thermal expansion coefficient of 0.5 × 10 ⁻⁶ / K or more and the heat dissipating element. A substrate in which at least a part of the surface in contact with the heating element is SiC, and a carbon nanotube layer formed on the entire surface or a part of the surface or both surfaces of the substrate, The formed carbon nanotube layer is brought into contact with a heating element.
If such a heat spreader is inserted between the heating element and the cooling body, even if the thermal expansion coefficients of the heating element and the cooling body are different, the flexible carbon nanotube layer can easily relieve the thermal stress, and the thermal expansion coefficient can be reduced. There is an advantage that stress breakage and crack generation due to the difference hardly occur.

  By the way, the heat spreader using the carbon nanotube is joined to a semiconductor element or a heat sink through brazing, Au-Sn plating, a soldering process, or the like. That is, the carbon nanotube comes into contact with these bonding materials. At that time, in each process, the liquid bonding material comes into contact with the tip of the carbon nanotube layer, and the bonding is performed. However, not only the contact with the tip but also the bonding material is placed in the pores in the carbon nanotube layer. It will invade to some extent. As a result, the flexibility of the carbon nanotube layer is lowered, and the thermal stress relaxation performance may be lowered.

Therefore, the present inventor completed the present invention by making further improvements so that the thermal stress relaxation ability does not decrease no matter what kind of bonding material is used. That is, as a result of intensive studies to increase the thermal stress relaxation function without the penetration of the bonding component into the pores of the carbon nanotube layer, by forming a boron nitride (BN) nanotube layer instead of the carbon nanotube, It has been found that the molten metal does not enter the pores and becomes a heat spreader having a high thermal stress relaxation function.
The heat dissipation method according to the present invention is characterized by having the following configuration.

(1) The heat dissipating method according to the present invention is a heat dissipating method in which a heat generating body having a difference in thermal expansion coefficient of 0.5 × 10 ⁻⁶ / K or more and a heat dissipating body are brought into contact to transmit heat from the heat generating body to the heat dissipating body. The radiator has a substrate in which at least a part of a surface in contact with the heating element is SiC, and a BN nanotube layer formed on the entire surface or a part of the surface or both surfaces of the substrate. The BN nanotube layer formed on the substrate surface of the body is brought into contact with the heating element.
(2) In another heat dissipation method according to the present invention, a heat spreader is interposed between a heat generating body having a difference in thermal expansion coefficient of 0.5 × 10 ⁻⁶ / K or more and the heat dissipating body to change from the heat generating body to the heat dissipating body. In the heat dissipation method for transferring heat, the heat spreader includes a substrate in which at least a part of the substrate surface is SiC, and a BN nanotube layer formed on the entire surface or a part of one or both surfaces of the SiC substrate, The BN nanotube layer formed on the substrate surface of the heat spreader is brought into contact with a heating element and / or a radiator.

(3) The heat dissipation method according to the above (1) or (2), wherein the BN nanotube layer is brought into contact with a heating element and / or a heat dissipation element through a bonding material.
(4) The heat dissipation method according to (3) above, wherein the bonding material is any one of In—Sn, Au—Sn, solder, or active silver iron.
(5) The heat dissipation method according to any one of (1) to (4) above, wherein the substrate is SiC.

(6) The heat dissipation method according to any one of (1) to (5) above, wherein a difference in coefficient of thermal expansion between the heating element and the heat dissipation element is 1 × 10 ⁻⁶ / K or more. It is characterized by that.
(7) The heat dissipation method according to any one of (1) to (6), wherein the heating element is a semiconductor.
(8) The heat dissipation method according to (7), wherein the main component of the heating element is any one of Si, InP, GaAs, GaN, and AlN.

(9) The heat dissipation method according to any one of (1) to (7), wherein the BN nanotubes are grown substantially perpendicular to the substrate surface in the BN nanotube layer. .
(10) The heat dissipation method according to any one of (1) to (9) above, wherein a length of the BN nanotube is 1 μm or more.
(11) The heat dissipation method according to the above (10), wherein the BN nanotube has a length of 10 μm or more.

  Since BN nanotubes have the same high thermal conductivity as carbon nanotubes, and have the characteristics that they are very likely to be formed, if a layer of BN nanotubes is formed on the surface of the heat spreader, even if a heating element and a radiator ( Even if thermal stress is generated during the cooling body), the BN nanotube layer can relieve the stress. Moreover, since the material BN itself has very low wettability with various liquids, when the BN nanotube is brought into contact with the molten metal, the molten metal is extremely difficult to enter the pores in the BN nanotube layer. Therefore, the nanotube layer can maintain sufficiently high flexibility even after the molten metal is solidified after bonding.

  The heat dissipation method according to the present invention can be preferably used for, for example, a heat dissipation method for semiconductor devices and a heat dissipation method for laser diodes. In addition, in the case of a heat dissipation method for heat dissipation by combining an existing heat generator and a heat radiator, a heat dissipation method using a heat spreader in which BN nanotube layers are provided on both surfaces of the substrate is preferable.

  The heat dissipating method according to the present invention uses a heat dissipating body or heat spreader in which BN nanotubes having excellent flexibility and flexibility are formed on the surface. By inserting and using, it is possible to relieve the thermal stress between the two, produce no peeling and the like, and heat can easily propagate.

  The heat dissipating method according to the present invention relates to a heat dissipating method for transferring heat from the heat generating element to the heat dissipating element by contacting the heat generating element and the heat dissipating element having different thermal expansion coefficients. In the present invention, the heat dissipator has a substrate in which at least a part of the surface in contact with the heat generator is SiC, and a BN nanotube layer formed on the entire surface or a part of the contact surface or both surfaces of the substrate. The BN nanotube layer formed on the substrate surface of the radiator is brought into contact with the heating element. At this time, the substrate of the heat dissipating body has a flat plate shape and may have a BN nanotube layer formed on both surfaces, or a so-called heat sink shape having fins on one surface, and is in contact with the heating element. A BN nanotube layer may be formed on the surface. In this case, the heat spreader of the present invention also serves as a heat sink (see FIG. 2A). In the figure, BNNT means a BN nanotube.

Furthermore, in the heat dissipation method according to the present invention, a heat spreader is inserted between the heat generating element having a difference in thermal expansion coefficient of 0.5 × 10 ⁻⁶ / K or more and the heat dissipating element to heat the heat generating element to the heat dissipating element. It is about the heat dissipation method to convey. And the said heat spreader used for the thermal radiation method concerning this invention has a board | substrate whose at least one part of a substrate surface is SiC, and a BN nanotube layer formed in the whole surface or part of this SiC substrate surface, It is characterized by the above-mentioned. To do. As a result, as shown in FIG. 2B, the BN nanotube layer formed on the substrate surface of the heat spreader is brought into contact with the heating element and the radiator, regardless of the difference in the thermal expansion coefficient between the heating element and the radiator. The thermal stress generated between them can be relaxed satisfactorily. FIG. 2C is a diagram showing a heat dissipation method using a conventional heat spreader. In this heat dissipation method, a problem occurs due to thermal stress generated at the interface (bonding layer) under the temperature cycle as described above.
In FIGS. 2A to 2C, the SiC substrate is insulative so that the semiconductor substrate is directly bonded to the SiC heat spreader. However, in an actual device, when SiC is non-insulating, In order to ensure insulation between the semiconductor substrate and the SiC heat spreader, the AlN plate may be bonded to the heat spreader after the semiconductor substrate is bonded to an insulating ceramic plate such as AlN.

  As will be described later, the substrate may be made of SiC only at the portion where the BN nanotubes are formed, but the substrate itself is preferably made of SiC from the viewpoint of heat transfer and cost. In the case of a substrate other than SiC, SiC may be coated on at least a part of the substrate surface. The BN nanotubes are preferably grown substantially perpendicular to the substrate surface. As a result, the tip portion of the BN nanotube comes into contact with the unevenness of the surface of the counterpart material such as a heating element without gaps, and the thermal resistance can be reduced, and even if the counterpart material bends due to thermal stress, The tip of the BN nanotube can flexibly follow the surface of the counterpart material.

BN nanotubes are obtained by converting carbon nanotubes.
Carbon nanotubes can be produced, for example, by the method described in Patent Document 2 (hereinafter also referred to as sublimation method). In other words, it may be heated to a temperature at which the SiC substrate is decomposed and silicon atoms are lost under vacuum. When SiC is heated under vacuum, for example, when the vacuum degree is 10 ⁻⁷ torr and SiC reaches 1400 ° C., SiC decomposes and silicon atoms are lost. At this time, since silicon atoms are lost in order from the surface of the SiC crystal, the surface of the SiC crystal is first changed to a layer deficient in silicon atoms, and the thickness of the Si removal layer gradually penetrates into the original SiC crystal. Increase. When this layer is observed with a microscope, it can be seen that the carbon nanotubes are formed vertically from the SiC surface.

  As the SiC substrate, a SiC wafer used as a substrate for a semiconductor device can be used, or a substrate obtained by growing a SiC thin film on a substrate surface other than SiC may be used. At this time, the substrate or the outermost SiC thin film is preferably grown in the <0001> direction. When growing with other orientations, the orientation of the carbon nanotubes produced may not be aligned.

Next, the carbon nanotubes are reacted with an atmosphere containing boron and nitrogen to obtain BN nanotubes.
For example, a carbon nanotube can be converted into boron nitride while leaving its original form by chemically reacting the carbon nanotube with a boron oxide such as B ₂ O ₃ and nitrogen at a high temperature (Cited document 3). . B ₂ O ₃ decomposes at a high temperature to generate B ₂ O ₃ gas, B ₂ O ₂ gas, BO ₂ gas, etc., reaches the carbon nanotubes, and undergoes reduction by the carbon of the nanotubes and simultaneously reacts with nitrogen To generate BN. By this reaction, BN nanotubes can be obtained in the crucible while leaving the form of the raw material carbon nanotubes.

  The boron source may be another substance as long as it is a substance that generates boron oxide by heating. For example, it may be an organic boric acid compound such as boric acid or melamine borate, a solid or liquid substance such as a mixture of boric acid and an organic substance, or a gas containing boron or oxygen. The nitrogen source may be a neutral or reducing gas containing nitrogen. Nitrogen, ammonia and the like are easy to use, and are used as they are, or mixed and diluted. Nitrogen gas is most preferred because it is inexpensive and safe.

  BN formation occurs thermodynamically above 1200 ° C. The reaction temperature is preferably 1200 ° C. to 2100 ° C., particularly preferably 1300 ° C. to 1800 ° C. If the temperature is too high, the crystallization of BN proceeds and plate-like crystals are generated, so that the shape of the nanotube cannot be maintained. Therefore, the upper limit is 2100 ° C. or lower, preferably 1900 ° C. or lower. In addition, since there is a greater tendency for BN to crystallize and form plate crystals as the atmosphere has more oxygen, the temperature is preferably 1800 ° C. or less, preferably 1600 or less in an oxygen-rich environment.

As described above, carbon nanotubes can be converted into BN nanotubes.
In order to reduce the contact thermal resistance by infiltrating fine irregularities present on the surface of the counterpart material, it is effective if the structure before the treatment is a carbon nanotube in which at least about 1 μm of the outermost surface is vertically aligned. When the surface roughness of the counterpart material is low and the flatness is low, the thickness of the layer containing carbon nanotubes is preferably 10 μm or more. The inclusion of carbon nanotubes includes that a layer made of porous carbon is formed inside vertically aligned carbon nanotubes formed on the outermost surface. At the time of contact, the porous layer is deformed and deformed in accordance with the surface shape of the counterpart material, and the contact property is increased.
The carbon nanotubes and porous carbon described above are converted into BN nanotubes and porous BN, respectively.

In the present invention, the substrate may be other than SiC, and an AlN or Si ₃ N ₄ substrate may be used in order to ensure insulation including the substrate. When these substrates are used, for example, a SiC layer is coated on the substrate in advance, and the SiC layer is converted into a carbon nanotube layer by the above-described thermal decomposition method (sublimation method) of SiC. What is necessary is just to convert into a layer. The BN nanotubes may be formed on the entire surface of the substrate, or may be formed only at a portion where the BN nanotube is bonded to the heat generator or the heat radiator.
Further, the carbon nanotube formation method is not limited to this, and the carbon nanotube may be formed by a method such as a CVD method.

  In the heat spreader used in the heat dissipation method according to the present invention, BN nanotubes converted from carbon nanotubes are grown directly from the substrate surface. In addition, since the lattice matching with the substrate is well, it can be said that the thermal resistance between the substrate and the BN nanotube is substantially zero, and has a high thermal conductivity. This is because, for example, when BN nanotubes are formed on the substrate surface by vapor phase synthesis or when the BN nanotubes are dispersed in a liquid and arranged vertically on the substrate surface by some method. It is a very different feature. In these cases, it is considered that a thermal resistance always exists between the carbon nanotube and the substrate.

  The heat dissipation method according to the present invention is preferably used when the heating element is a semiconductor. This is particularly effective when the main constituent material of the semiconductor as the heating element is Si, InP, GaAs, GaN, or AlN.

The heat spreader in which the BN nanotubes are formed and the semiconductor element may be bonded by a conventional bonding method using Au-Sn or solder. The BN nanotube and the heat sink may be joined in the same manner, or may be joined by brazing with In—Sn iron or active silver iron. BN nanotubes converted from carbon nanotubes produced by the sublimation method have a high production density, and even when brazed, the components of the bonding material do not easily penetrate into the pores of the BN nanotubes and have a high thermal stress relaxation function even after bonding. Can demonstrate. For this reason, the bonding material is not particularly limited, and a known material such as In—Sn, Au—Sn, solder, or active silver iron can be preferably used.
Further, the joining to the heat sink side may be screwed so that the BN tube and the heat sink are in direct contact without using brazing. Since BN nanotubes have a high contact property, a sufficiently low thermal resistance can be obtained just by bringing them into contact with each other, so that heat dissipation is not inhibited.

The heat spreader used in the heat dissipating method according to the present invention is very effective when the difference in thermal expansion coefficient at room temperature between the heat generating body and the heat dissipating body (cooling body) is 1 × 10 ⁻⁶ / K or more. For example, if the heating element is a semiconductor element, for example, each InP and thermal expansion coefficient of GaAs is because it is ^{4.3 × 10 -6 /K,5.9×10 -6 / K} , using the Cu heat sink If the effect is tremendous. The use of CuW (thermal conductivity of about 200 W / mK) with a thermal expansion coefficient of 6.5 × 10 ⁻⁶ / K instead of the Cu heat sink is effective for an InP device. In the case of a GaAs device, there is no problem in terms of thermal stress even if GaAs is directly bonded to CuW, but the heat dissipation performance is improved because SiC has higher thermal conductivity.

  The length of the BN nanotubes required in the BN nanotube layer is of course a semiconductor element having a very high surface roughness, although the length varies depending on the surface roughness and surface waviness of the counterpart material such as a heat radiator. In this case, the thermal stress can be alleviated if the length of the BN nanotube is 1 μm or more. If it is less than 1 μm, the effect is small. When the length is 10 μm or more, the thermal stress can be almost alleviated regardless of the mating material.

(1) Material <Semiconductor substrate>
As the semiconductor element material, InP, GaAs, Si, and GaN having a size of 10 × 10 mm and a thickness of 0.25 mm were used.
<Heatsink>
As the heat sink, 10 × 10 mm and 1 mm thick Cu and CuW were used.
<Heat spreader>
As the heat spreader, various materials shown in Table 1 having a size of 10 × 10 mm and a thickness of 0.35 mm were used.

<Production of heat spreader with carbon nanotube layer>
In the case of using a substrate other than SiC, a substrate in which an SiC film was previously coated with various thicknesses by a CVD method was used. Various substrates were placed in a vacuum furnace and heated at various temperatures, times, and degrees of vacuum to generate carbon nanotubes on both surfaces of the substrate (sublimation method).
Thereby, a carbon nanotube layer was formed on the substrate surface.

<Conversion to BN nanotubes>
As shown in FIG. 1, a graphite crucible with an inner diameter of 2 cm and a depth of 2 cm was placed in an external heating type thermal CVD furnace having a carbon furnace core tube with an inner diameter of 200 mm, and ₃ g of B ₂ O ₃ powder was loaded. A carbon nanotube sample was placed thereon. N ₂ or NH ₃ gas was introduced at 2.0 liters / min, heated at a temperature of 1530 ° C. for 30 minutes, and then naturally cooled.
From the SEM observation of the sample, the same nanotubes as before the treatment could be confirmed. From the powder method X-ray diffraction, the recovered sample was BN having a layered structure.

(2) Assembly After applying about 10 μm of In—Sn 鑞 on the surface of the semiconductor substrate and the heat sink, sandwich the heat spreader or the heat spreader with the carbon nanotube layer on the surface where the In—Sn 鑞 is applied, and in vacuum at 550 ° C. for 10 minutes. After heating, the mixture was cooled to room temperature at a rate of 0.5 ° C./min to obtain a joined body.

(3) Evaluation <Temperature cycle test>
Each sample was put into a furnace, heated to 300 ° C. at a rate of temperature increase of 10 ° C./min in 1 atm of nitrogen, then held for 1 minute, then taken out of the furnace and put into water. This was repeated 20 times.
After each treatment, the joint of the sample was observed visually or with a microscope, and when a crack occurred, the rapid cooling test was stopped at that time.

<Result>
The results are shown in Table 1. In the table, CNT means a carbon nanotube.

Samples 1 to 9 on which the carbon nanotubes were formed finished the temperature cycle test halfway due to the occurrence of cracks. Samples 10 and 16 to 18 on which BN nanotubes were formed cleared 20 times. In addition, when the raw materials of the semiconductor element, the heat sink, and the heat spreader are the same, it has been found that converting carbon nanotubes to BN nanotubes is superior in durability (thermal stress relaxation performance). Furthermore, the longer the BN nanotubes, the better the durability.
As a result of observing the cross section of the joint after the test, it was found that a large amount of In—Sn 鑞 was soaked in the carbon nanotube layer. In contrast, In—SnＳ was hardly soaked into the BN nanotube layer. The reason why the present invention showed excellent thermal shock resistance is considered to be because the In—Sn soot component hardly penetrates into the BN nanotube layer, and the BN nanotube layer has high flexibility and high thermal stress relaxation function.

It is the schematic of the apparatus which synthesize | combines the BN nanotube used in the Example. (A) It is a figure showing the outline of the thermal radiation method concerning this invention. (B) It is a figure which shows another outline of the thermal radiation method which concerns on this invention. (C) It is a figure showing the outline of the heat dissipation method in the past.

Claims (11)

In a heat dissipation method for transferring heat from a heating element to a radiator by contacting the heating element and the radiator with a difference in thermal expansion coefficient of 0.5 × 10 ⁻⁶ / K or more,
The radiator includes a substrate in which at least a part of a surface in contact with the heating element is SiC, and a boron nitride (BN) nanotube layer formed on the entire surface or a part of the surface or both surfaces of the substrate. ,
A heat dissipating method comprising contacting a BN nanotube layer formed on a substrate surface of the heat dissipating member with a heat generating member. In a heat dissipation method in which a heat spreader is interposed between a heat generating body having a difference in thermal expansion coefficient of 0.5 × 10 ⁻⁶ / K or more and a heat dissipating body to transfer heat from the heat generating body to the heat dissipating body,
The heat spreader has a substrate in which at least a part of the substrate surface is SiC, and a BN nanotube layer formed on the entire surface or a part of one or both surfaces of the SiC substrate,
A heat dissipation method comprising contacting a BN nanotube layer formed on a substrate surface of the heat spreader with a heat generator and / or a heat radiator.   The heat dissipation method according to claim 1 or 2, wherein the BN nanotube layer is brought into contact with a heating element and / or a heat dissipation element via a bonding material.   The heat dissipation method according to claim 3, wherein the bonding material is any one of In—Sn, Au—Sn, solder, or active silver iron.   The heat dissipation method according to any one of claims 1 to 4, wherein the substrate is SiC. Radiating method according to any one of claims 1 to 5, wherein the difference in thermal expansion coefficient at room temperature of the heating element and the heat dissipating body is 1 × 10 ^-6 / K or more.   The heat dissipation method according to claim 1, wherein the heating element is a semiconductor.   The heat dissipating method according to claim 7, wherein a main constituent material of the heating element is any one of Si, InP, GaAs, GaN, and AlN.   The heat dissipation method according to claim 1, wherein in the BN nanotube layer, BN nanotubes are grown substantially perpendicular to the substrate surface.   The heat dissipation method according to claim 1, wherein the BN nanotube has a length of 1 μm or more.   The length of the said BN nanotube is 10 micrometers or more, The heat dissipation method of Claim 10 characterized by the above-mentioned.

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