JP2005311261A - Heat radiating plate made of silicon carbide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat radiating plate with an excellent heat conductivity which can be suitably used as a substrate for modules for loading a semiconductor device having a higher releasing value. <P>SOLUTION: In the heat radiating plate made of silicon carbide obtained by processing silicon carbide monocrystal or silicon carbide polycrystal produced by a sublimation recrystallization method, the heat conductivity of the heat radiating plate at an ordinary temperature is 200 to 550 W/mK. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a high thermal conductive heat dissipation plate that can be suitably used as a heat sink for dissipating heat energy generated from a heat source.

  With the increasing information density, the processing capacity of electronic components has improved remarkably. For this reason, a large amount of heat is generated from each component. In order to stably operate these electronic components, it is preferable to maintain a constant temperature, and various devices have been made for cooling the electronic components. Typically, high temperature electronic components are mounted and used on “heat sinks, materials that can absorb heat, components or devices that use such materials to thermally protect the system” Is common.

  Examples of materials that have been put into practical use as heat sink materials from early on are metals or metal alloys with good thermal conductivity such as Cu and Cu-W, or semiconductor or insulating properties such as silicon carbide (SiC) and aluminum nitride (AlN). However, it has been found that there is a limit to cooling the heat generated by improving the performance of electronic components using a heat sink made of only such materials. Heat sink material is needed. As a heat sink material, SiC is one of the most frequently used materials (for example, Patent Document 1).

However, the normally used SiC of ceramic sintered body has a room temperature thermal conductivity of 100 to 130 W / mK, which is not a sufficient value. SiC itself has a thermal conductivity next to diamond among substances existing in the world, and the thermal conductivity specific to the material exhibits a thermal conductivity of about 500 W / mK. The reason why sufficient heat conductivity cannot be obtained with the ceramic heat sink is that the ceramic sintered body is an aggregate of fine crystal grains, and there are many grain boundaries in the material. Furthermore, a sintering aid is added to the ceramic sintered body to assist sintering during production, but this aid may form a grain boundary and deteriorate thermal conductivity. . Grain boundaries scatter phonons (quantities of crystal lattice vibration), which are responsible for heat conduction, and cause a significant decrease in thermal conductivity.
Japanese Patent Laid-Open No. 2003-100939

  As described above, SiC of a ceramic sintered body that has been conventionally used has a small room temperature thermal conductivity of 100 to 130 W / mK, and cannot play a sufficient role as a heat sink material for electronic components. This is because the ceramic sintered body is an aggregate of fine crystal grains, and there are many grain boundaries in the material.

  Therefore, an object of the present invention is to provide a SiC high thermal conductivity heat dissipation plate having fewer crystal grain boundaries as a heat sink material than ceramic sintered body SiC.

The present invention
(1) A heat sink processed from a silicon carbide single crystal or silicon carbide polycrystal produced by a sublimation recrystallization method, and the heat conductivity of the heat sink is 200 to 550 W / mK at room temperature. A heat sink made of silicon carbide,
(2) The silicon carbide heat sink according to (1), wherein the heat sink has a resistivity of 10 ⁸ to 10 ²² Ωcm.
(3) The heat sink made of silicon carbide according to (1), wherein no through-hole defect exists in the heat sink,
It is.

  According to the present invention, a high-resistivity, high-quality SiC single crystal or SiC polycrystal can be grown with good reproducibility by an improved Rayleigh method using a seed crystal. If a SiC single crystal or polycrystalline wafer cut out from such a crystal is used, a heat radiating plate having excellent heat conduction characteristics can be manufactured.

  First, the heat sink made of silicon carbide single crystal or polycrystal of the present invention will be described. The silicon carbide single crystal or polycrystal constituting the heat sink of the present invention is produced by a sublimation recrystallization method, and therefore has no or very few crystal grain boundaries in the material, and the room temperature thermal conductivity is 200 to 550 W / mK. It is characterized by saying. When the normal temperature thermal conductivity is less than 200 W / mK, for example, when a power module equipped with an IGBT element is actually operated, the temperature of the IGBT element is 70 ° C., which is close to the upper limit of the operating temperature of the IGBT element. The problem is that the stable operation of the system may be hindered. Note that 550 W / mK is the maximum thermal conductivity obtained from the physical properties of SiC.

  Next, the sublimation recrystallization method will be described. The sublimation recrystallization method is a method for producing SiC crystals by sublimating SiC powder at a high temperature exceeding 2000 ° C. and recrystallizing the sublimation gas into a low temperature part. In this method, a method for producing an SiC single crystal using a seed crystal composed of an SiC single crystal is particularly called an improved Rayleigh method (Yu. M. Tailov and VF Tsvetkov, Journal of Crystal Growth, vol. 52 (1981) pp. 146-150), it is used for the production of bulk SiC single crystals. In the modified Rayleigh method, a seed crystal is used, so that the nucleation process of the crystal can be controlled, and the atmospheric pressure is controlled from about 100 Pa to about 15 kPa with an inert gas, so that the crystal growth rate and the like can be reproducible. I can control it. The principle of the improved Rayleigh method will be described with reference to FIG. The SiC single crystal used as a seed crystal and the SiC crystal powder used as a raw material (usually used after cleaning and pre-treatment of an abrasive prepared by the Acheson method) are used in a crucible (usually made of graphite). And heated to 2000 to 2400 ° C. in an inert gas atmosphere such as argon (133 Pa to 13.3 kPa). At this time, the temperature gradient is set so that the seed crystal has a slightly lower temperature than the raw material powder. After sublimation, the raw material is diffused and transported in the direction of the seed crystal by a concentration gradient (formed by a temperature gradient). Single crystal growth is realized by recrystallization of the source gas that has arrived at the seed crystal on the seed crystal.

  In a conventional heat sink using a SiC ceramics sintered body, as described above, a large number of crystal grain boundaries exist in the material, and phonons (quantities of crystal lattice vibration), which are responsible for heat conduction, are scattered. The thermal conductivity is greatly reduced. In contrast, in SiC single crystals, there are no or very few crystal grain boundaries, so there is no decrease in thermal conductivity, and extremely excellent thermal conductivity can be maintained.

  In the above-described sublimation recrystallization method, a seed crystal having a crystal orientation of {0001} plane of a SiC single crystal is usually used, but the produced SiC single crystal has a through hollow defect having a diameter of about 0.1 to 10 μm. (Commonly known as micropipe defects). This penetrating hollow defect is a kind of large-sized screw dislocations existing in the crystal, and it is considered that the core of the dislocations is generated in a cavity in order to alleviate a large strain caused by the dislocations.

  When considering use of a semiconductor element as a heat sink, the presence of the above-described through-hole defect in the heat sink may be undesirable as a heat sink. As an example, when joining the heat sink to the element module using a metallic brazing material, there may be a problem that the brazing material penetrates the hole to form a conductive path.

  In such a case, the through hollow defect obtained by using a SiC seed crystal having a specific orientation ({11-20} plane and {1-100} plane) as the plane orientation of the SiC seed crystal used for growth It is preferable to use a crystal free from any. It is disclosed in Japanese Patent Application Laid-Open No. 5-262599 that the growth using these plane orientations can completely prevent the occurrence of through hollow defects. As a mechanism for preventing the occurrence of through-hollow defects, the growth mechanism is not due to step supply from the screw dislocations in the conventional {0001} plane growth, and the occurrence of through-hole defects, which is a kind of large screw dislocations, It is completely suppressed. In this way, a SiC single crystal heat dissipation plate having no holes is obtained.

Further, depending on the type of semiconductor element formed on the heat sink, there may be a case where the heat resistance of the heat sink needs to be sufficiently high. For example, when a lead frame on which a chip of a high-power IGBT element is mounted is directly attached to a heat sink, it is essential that the heat sink itself has sufficient insulation. In this case, a method using a wafer cut out from single crystal SiC having semi-insulating properties is effective. Single-crystal SiC having semi-insulating properties can be produced by using a high-purity silicon carbide raw material used for crystal growth. As the high purity raw material, for example, a powder obtained by a chemical vapor deposition method (CVD method) using a Si compound gas (for example, SiH ₄ ) and a C compound gas (for example, C ₃ H ₈ ) can be used. The resistivity of the wafer obtained from the SiC single crystal produced using this raw material is 10 ⁸ to 10 ²² Ωcm. When the resistivity is less than 10 ⁸ Ωcm, the insulation between the elements becomes insufficient, which may hinder the operation of the elements. The resistivity of 10 ²² Ωcm is the maximum resistivity obtained from the physical properties of SiC.

  On the other hand, it is also possible to use polycrystalline SiC instead of the above-described single crystal SiC as the heat sink material. In general, polycrystals are inferior in heat conduction characteristics because there are more grain boundaries as compared with single crystals. However, the SiC polycrystal produced by the sublimation recrystallization method has the characteristics that the grain size is large (0.5 to several mm diameter) and the grain boundary density is small. For this reason, SiC polycrystal having sufficiently large thermal conductivity (200 W / mK or more) can be obtained as compared with the SiC ceramic sintered body, and can be sufficiently used as a heat sink. Furthermore, since polycrystalline SiC does not need to prepare a seed crystal as compared with single-crystal SiC, it has the advantage that it is easier to manufacture and can be manufactured at a lower cost.

(Example 1)
Examples of the present invention will be described below. First, the single crystal growth apparatus shown in FIG. 2 will be briefly described. Crystal growth is performed by sublimating the SiC crystal powder 2 and recrystallizing it on the SiC single crystal 1 used as a seed crystal. The seed crystal SiC single crystal 1 is attached to the inner surface of the lid 4 of the high-purity graphite crucible 3. The raw material SiC crystal powder 2 is filled in a high-purity graphite crucible 3. Such a graphite crucible 3 is installed inside a double quartz tube 5 by a support rod 6 made of graphite. Around the graphite crucible 3, a graphite felt 7 for heat shielding is installed. The double quartz tube 5 can be high vacuum evacuated (10 ⁻³ Pa or less) by a vacuum evacuation device, and the internal atmosphere can be pressure controlled by Ar gas. In addition, a work coil 8 is provided on the outer periphery of the double quartz tube 5, and the graphite crucible 3 can be heated by flowing a high-frequency current to heat the raw material and the seed crystal to a desired temperature. The temperature of the crucible is measured using a two-color thermometer by providing an optical path having a diameter of 2 to 4 mm at the center of the felt covering the upper and lower parts of the crucible and extracting light from the upper and lower parts of the crucible. The temperature at the bottom of the crucible is the raw material temperature and the temperature at the top of the crucible is the seed temperature.

  Next, an example of manufacturing a SiC single crystal using this crystal growth apparatus will be described. First, a hexagonal SiC single crystal wafer having a (0001) face with a diameter of 50 mm was prepared as a seed crystal 1. Next, the seed crystal 1 was attached to the inner surface of the lid 4 of the graphite crucible 3. The inside of the graphite crucible 3 was filled with SiC crystal raw material powder 2 produced by the Atchison method. Next, the graphite crucible 3 filled with the SiC raw material was closed with the lid 4 and covered with the graphite felt 7, and then placed on the graphite support rod 6 and installed inside the double quartz tube 5. And after evacuating the inside of a quartz tube, the electric current was sent through the work coil and the raw material temperature was raised to 2000 degreeC. Thereafter, high-purity Ar gas was introduced as the atmospheric gas, and the raw material temperature was raised to the target temperature of 2400 ° C. while maintaining the pressure in the quartz tube at about 80 kPa. The growth pressure was reduced to 1.3 kPa over about 30 minutes, and then the growth was continued for about 20 hours. At this time, the temperature gradient in the crucible was 15 ° C./cm, and the growth rate was about 0.8 mm / hour. The diameter of the obtained crystal was 51 mm, and the height was about 16 mm.

When the silicon carbide single crystal thus obtained was analyzed by X-ray diffraction and Raman scattering, it was confirmed that a hexagonal SiC single crystal was grown. For the purpose of measuring the thermal conductivity of the crystal, four wafers having a thickness of 2.8 mm were cut out from the grown single crystal ingot. When the wafer was directly observed with an optical microscope, it was confirmed that there were about 10 to 100 penetrating hollow defect densities / cm ² . For further confirmation, etching was performed in a potassium hydroxide solution melted at 520 ° C. for 5 minutes. When this etching process is performed, the penetrating hollow defects can be observed as hexagonal etch pits. When the etch pit number density of the wafer was examined, through-hole defects having the same number density as in the visual observation were detected.

  For these wafers, the laser flash method (irradiating a sample with a thickness of 0.5 to 3 mm with laser light and analyzing the temperature history curve on the back surface determines the thermal diffusivity and specific heat, and calculates the thermal conductivity from the formula. When the thermal conductivity was measured by the calculation method), all the wafers showed a high thermal conductivity of 450 to 500 W / mK. Moreover, when electrical resistance measurement was implemented, the resistivity of 0.2-0.4 ohm-cm was shown.

  Next, the wafer obtained by cutting the SiC single crystal ingot by a conventionally known method was subjected to a chamfering process, followed by polishing and the like, thereby completing the production of a heat radiation plate having a length of 35 mm, a width of 30 mm, and a thickness of 2.5 mm.

  Next, as an insulating substrate having a conductor circuit or the like, an aluminum nitride substrate having a thickness of 4 mm in which a conductor circuit made of aluminum is bonded to one surface and an aluminum plate is bonded to the other surface using a brazing material is obtained. The heat radiation plate and the aluminum nitride substrate thus obtained were joined by heating and pressure bonding with the aluminum plate interposed therebetween, and the production of the power module substrate was completed.

  Next, an IGBT element was mounted on this module substrate, and a heat module was attached to the cooling unit to produce a power module. The power module was actually operated, and the temperature of the IGBT element was measured. As a result, the element temperature after the start of operation was kept in the range of 45 ° C. ± 5 ° C., and was kept at a sufficiently low temperature with respect to 70 ° C. which is the stable operation upper limit value of the IGBT element.

(Example 2)
In the same manner as in Example 1, a seed crystal 1 having (11-20) and (1-100) planes cut out from a hexagonal SiC single crystal ingot having a (0001) plane was prepared as a seed crystal. . Next, the seed crystal 1 was attached to the inner surface of the lid 4 of the graphite crucible 3. The graphite crucible 3 was filled with high-purity SiC crystal powder 2 obtained by the CVD method. Next, the graphite crucible 3 filled with the SiC raw material was closed with the lid 4 and covered with the graphite felt 7, and then placed on the graphite support rod 6 and installed inside the double quartz tube 5. And after evacuating the inside of a quartz tube, the electric current was sent through the work coil and the raw material temperature was raised to 2000 degreeC. Thereafter, high-purity Ar gas was introduced as the atmospheric gas, and the raw material temperature was raised to the target temperature of 2400 ° C. while maintaining the pressure in the quartz tube at about 80 kPa. The growth pressure was reduced to 1.3 kPa over about 30 minutes, and then the growth was continued for about 20 hours. At this time, the temperature gradient in the crucible was 15 ° C./cm, and the growth rate was about 0.75 mm / hour. The diameter of each crystal obtained was 51 mm and the height was about 15 mm.

When the silicon carbide single crystal thus obtained was analyzed by X-ray diffraction and Raman scattering, it was confirmed that both had grown a hexagonal SiC single crystal. For the purpose of measuring the thermal conductivity of the crystal, four wafers having a thickness of 2.8 mm were cut from each grown single crystal ingot. When the wafer was directly observed with an optical microscope, no through-hole defect was observed at all. As a precaution, in order to confirm the presence or absence of a penetrating hollow defect, the grown crystal was cut so that a {0001} plane appeared and etched in a potassium hydroxide solution melted at 520 ° C. for 5 minutes. As a result of observing this surface, no hexagonal etch pits due to through hollow defects were observed, and it was confirmed that there were no through hollow defects in the crystal. When the thermal conductivity of these wafers was measured by a laser flash method, all the wafers showed a high thermal conductivity of 450 to 500 W / mK. Moreover, when electrical resistance measurement was implemented, all showed a resistivity of 10 ¹⁰ Ωcm.

  Next, the wafer obtained by cutting the SiC single crystal ingot by a conventionally known method was subjected to a chamfering process, followed by polishing and the like, thereby completing the production of a heat radiation plate having a length of 35 mm, a width of 30 mm, and a thickness of 2.5 mm.

  Next, after directly brazing the lead frame on which the IGBT elements were wired to this heat radiating plate, the heat radiating plate was attached to the cooling unit to produce a power module and actually operated. When the operation of the element was confirmed, it was stable without any malfunction due to current leakage. From this result, since there is no through-hole defect in the heat sink, the generation of conductive paths in the heat sink can be prevented, and direct wiring on the heat sink is possible (the module manufacturing process can be simplified). Was confirmed. Further, when the temperature of the IGBT element was measured, the element temperature after the start of operation was kept in a range of 35 ° C. ± 5 ° C., and was kept at a sufficiently low temperature with respect to the stable operation upper limit of 70 ° C. It was.

(Example 3)
The procedure was the same as Example 1 except that no seed crystal was used. The inside of the graphite crucible 3 was filled with SiC crystal raw material powder 2 produced by the Atchison method. Next, the graphite crucible 3 filled with the SiC crystal raw material was closed with the lid 4 and covered with the graphite felt 7, and then placed on the graphite support rod 6 and installed inside the double quartz tube 5. And after evacuating the inside of a quartz tube, the electric current was sent through the work coil and the raw material temperature was raised to 2000 degreeC. Thereafter, high-purity Ar gas was introduced as the atmospheric gas, and the raw material temperature was raised to the target temperature of 2400 ° C. while maintaining the pressure in the quartz tube at about 80 kPa. The growth pressure was reduced to 1.3 kPa over about 30 minutes, and then the growth was continued for about 20 hours. At this time, the temperature gradient in the crucible was 15 ° C./cm, and the growth rate was about 1.0 mm / hour. The obtained polycrystal had a diameter of 51 mm and a height of about 20 mm.

  When the silicon carbide polycrystal thus obtained was analyzed by X-ray diffraction and Raman scattering, it was confirmed that the SiC polycrystal was grown. For the purpose of measuring the impurity concentration of the crystal, five wafers having a thickness of 2.8 mm were cut out from the grown single crystal ingot. When these wafers were measured for thermal conductivity by the laser flash method, all wafers were inferior to single-crystal SiC at 200 to 250 W / mK, but higher thermal conductivity than ceramic sintered bodies. Indicated. Moreover, the resistivity of the wafer obtained by the electrical resistance measurement showed a resistivity of 0.8-1 Ωcm.

  Next, in the same manner as in Example 1, a heat module was manufactured and a power module on which an IGBT element was mounted was formed.

  Next, the power module was actually operated, and the temperature of the IGBT element was measured. As a result, the element temperature increased after the start of operation, but was stable in the range of 55 ° C. ± 5 ° C. This temperature was about 10 ° C. higher than that of the single crystal shown in Example 1 (considered to be due to the difference in thermal conductivity), but this is the upper limit of the stable operating temperature of the IGBT element. The temperature was sufficiently lower than ° C.

(Comparative example)
When the thermal conductivity of a commercially available high-density SiC ceramic processed into a heat sink was measured by a laser flash method, the room temperature thermal conductivity was 125 W / mK. Using this heat radiating plate, a module substrate was formed in the same manner as in Example 1. Next, after mounting an IGBT element on this module substrate and attaching a heat sink to the cooling unit, the power module was actually operated, and the temperature of the IGBT element was measured. As a result, the device temperature kept in the range of 70 ° C. ± 5 ° C. after the operation started. This temperature is higher than in any of the above-described Examples 1 to 3. From this, the heat dissipation of the SiC single crystal heat sink or SiC polycrystalline heat sink is superior to the heat sink made of SiC ceramics. It was confirmed that the

It is a figure explaining the principle of an improved Rayleigh method. It is the schematic of the crystal growth apparatus used in the Example.

Explanation of symbols

1 seed crystal (SiC single crystal),
2 SiC crystal powder raw material,
3 crucibles (refractory metals such as graphite or tantalum),
4 Graphite crucible lid,
5 Double quartz tube,
6 Support rod,
7 Graphite felt (heat insulation),
8 Work coil,
9 High purity Ar gas piping,
10 Mass flow controller for high purity Ar gas,
11 Vacuum exhaust device.

Claims (3)

  A heat sink processed from a silicon carbide single crystal or silicon carbide polycrystal produced by a sublimation recrystallization method, wherein the heat conductivity of the heat sink at room temperature is 200 to 550 W / mK. A heat sink made of silicon carbide. The silicon carbide heat sink according to claim 1, wherein the heat sink has a resistivity of 10 ⁸ to 10 ²² Ωcm.   The silicon carbide heat sink according to claim 1, wherein no through-hole defect exists in the heat sink.

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