JP2578114B2 - Method for producing high thermal conductive aluminum nitride sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to an aluminum nitride sintered body and a method for producing the same, and more particularly, to a method for producing a dense aluminum nitride single phase having high thermal conductivity. And a method for producing the same.

(Prior art) Aluminum nitride (AlN) is used as a heat-resistant material because of its low strength reduction and high chemical resistance up to high temperatures, while utilizing its high-temperature conductivity and high electrical insulation to improve the performance of semiconductor devices. Promising as a heat sink material and an insulator material for circuit boards. Such aluminum nitride does not have a melting point under normal pressure and decomposes at a high temperature of 2500 ° C. or more, so it is used as a sintered body except for applications such as thin films.

Such an aluminum nitride sintered body is usually obtained by molding and sintering aluminum nitride and powder. Ultra fine powder (0.3μ
m or less), a dense sintered body can be obtained by itself, but oxygen in the oxide layer on the surface of the raw material powder is dissolved in the AlN lattice during sintering, or Al-O -N compound is produced, and as a result, the thermal conductivity of the sintered body without additive is at most
It is about 100w / mk. When AlN powder having a particle size of 0.5 μm or more is used, the sinterability is not good. Therefore, it is difficult to obtain a dense sintered body without any addition except by a hot press method. Therefore, when trying to obtain a sintered body at normal pressure,
Densification of sintered body and AlN of impurity oxygen in AlN raw material powder
In order to prevent solid solution in the grains, it is common to add a rare earth oxide, an alkaline earth metal oxide, or the like as a sintering aid (JP-A-60-127267, JP-A-60-127267). 1986-10
No. 071, JP-A-60-71575, etc.). These sintering aids are Al
It is thought that it reacts with the impurity oxygen of the N raw material powder to generate a liquid phase to achieve densification of the sintered body, and to fix this impurity oxygen as a grain boundary phase (oxygen trap) to achieve high thermal conductivity. I have.

By adding a sintering aid in this way, the sintered body surely becomes denser and has higher thermal conductivity, but on the other hand, as a result, the remaining grain boundary phase (sub-phase with respect to the main phase AlN phase) )The presence of,
Due to the presence of oxygen and the like that could not be completely trapped, that of aluminum nitride sintered body was about 190 w / mK
It was considerably lower than the theoretical thermal conductivity of 320 w / mK.

Therefore, various attempts have been made for the purpose of improving the thermal conductivity of the aluminum nitride sintered body, but none of them have been sufficiently satisfactory.

(Problems to be Solved by the Invention) At present, a material having a higher thermal conductivity is desired for a circuit board for mounting a semiconductor, a heat dissipation board, and the like. However, due to the presence of oxygen and other impurities, in particular, the grain boundary phase generated at the grain boundary as a result of the addition of the auxiliary agent, there has been a limit in increasing the thermal conductivity of the aluminum nitride sintered body.

The present invention has been made in consideration of the above points, and has as its object to provide an aluminum nitride sintered body having excellent thermal conductivity.

[Configuration of the invention]

(Means and Actions for Solving the Problems) To achieve the above object, the present inventors have studied the sintering aids added to aluminum nitride powder, sintering conditions, sintered body composition, sintered body microstructure, etc. As a result of conducting experiments and studies on the relationship between thermal conductivities, the following new matter was discovered, and the present invention was completed.

That is, when an alkaline earth metal compound and a rare earth compound are added to an AlN powder as additives, and baked for a long time in a reducing atmosphere containing nitrogen, a grain boundary phase (for example, Y-Al
-O compound, Ca-Al-O compound, etc.) were found to be reduced as compared with the conventional aluminum nitride sintered body. Then, it was found that when sintered for a sufficiently long time, an aluminum nitride sintered body having substantially no subphase and consisting of an AlN single phase and having a very high thermal conductivity as a polycrystal was obtained.

Based on this fact, the present invention is based on the results of various studies on the optimum conditions for achieving high thermal conductivity. A) Aluminum nitride as a main component, and an additive comprising an alkaline earth metal compound and a rare earth compound, The molded body or the molded body added with 0.05 to 25% in terms of the weight of each element is fired at 1550 to 2050 ° C for less than 4 hours,
A sintered body containing a constituent phase other than N is converted into a (b) nitrogen gas that embodies a reducing atmosphere by including a baking vessel that produces carbon gas and / or a substance that produces carbon gas during baking in the baking vessel. (C) the volume ratio of the internal volume of the firing vessel to the compact or sintered body is 1 ×
10 ⁰ to 1 × 10 ⁷ , baked at 1550 to 2050 ° C. for 12 hours or more, substantially free of constituent phases other than AlN, and having a density of 3.120 to
3. A method for producing a highly thermally conductive aluminum nitride sintered body, characterized in that the thermal conductivity at 25 ° C. exceeds 3.285 g / cm ³ and 230 W / m · k. In the aluminum nitride sintered body obtained by such a method, only the AlN crystal grains are observed when the constituent phases are observed using X-ray diffraction and an electron microscope, and other phases are not observed. When component analysis was performed, Al
It was a novel aluminum nitride sintered body in which N was the main component, the total amount of rare earth elements and alkaline earth elements was 10 to 3000 ppm, the content of impurity oxygen was 2000 ppm, and the content of other impurity cation elements was 1000 ppm or less.

The manufacturing method of the high thermal conductive aluminum nitride sintered body of the present invention is based on the aluminum nitride raw material powder, the type and amount of the additive, the amount of the additive, the firing atmosphere, and the firing temperature and time.

Aluminum nitride raw material powder, which is a main component, contains 7% by weight or less of oxygen in consideration of sinterability and thermal conductivity, and practically contains 0.01 to 7% by weight and has an average particle diameter of 0.05 to 5 μm. use.

Alkaline earth metal compounds and rare earth compounds are used as additives. As a compound of these elements, an oxide, a nitride, a fluoride, an oxyfluoride, an oxynitride, or a substance which becomes these compounds by firing is most suitable.
Examples of the substance that becomes an oxide upon firing include carbonates, nitrates, oxalates, and hydroxides of these elements.

These additives are added in an amount of 0.05 to 25% in terms of the weight of the alkaline earth metal and the rare earth element. If the addition amount is less than 0.05% by weight, the effect of the additive is not sufficiently exerted, and the sintered body is not densified, or oxygen is dissolved in the AlN crystal to obtain a high thermal conductive sintered body. . Also,
If the addition amount is excessively large, the grain boundary phase remains in the sintered body, and as a result, the effect of increasing the thermal conductivity may not be sufficiently expected.

In the method of the present invention, the molded body obtained by mixing the AlN powder and the additive as described above may be fired under the conditions described below,
In addition, as a constituent phase other than AlN produced by a conventional method (for example, Japanese Patent Application Laid-Open No. 61-117160) (alkaline earth metal element)
A sintered body containing an -Al-O compound, a (rare earth element) -Al-O compound, or the like may be used instead of the above-described molded body. The firing is performed in a reducing atmosphere containing nitrogen gas. The reducing atmosphere can be created by the presence of at least one or more of CO, H ₂ gas and carbon (gas phase and / or solid phase).

Of these, the simplest is to use a carbon container as the firing container. As for the firing vessel, aluminum nitride, alumina, Mo or the like is sufficient if the purpose is simply to obtain a sintered body (Japanese Patent Application Laid-Open No. 61-14769). However, in the case of using these containers, the (additive element) -Al-O compound phase or the like remains in the sintered body, and a sintered body with high thermal conductivity cannot be obtained.
In the present invention, a container that creates a carbon gas atmosphere during firing is used. Such a baking container is a container made of carbon as a whole, a container made of carbon as a whole and an AlN plate, a BN plate, a W plate or the like laid on a place where a sample is to be placed, or a container made of aluminum nitride and having an upper lid. An object made of carbon or the like can be used. The carbon gas atmosphere in the present invention is defined as 15
Vapor pressure of 1 × 10 ^{-6 to} 5 × 10 in the sintering temperature range of 50 to 2050 ° C
Refers to gas that generates about ^-2 Pa. This carbon gas has an effect of reducing the AlN sintered body during firing, and more specifically, a grain boundary phase such as an (additive element) -Al-O ternary compound is removed from the sintered body. The action of removing works, and the aluminum nitride sintered body becomes an AlN single phase and changes into a sintered body having high thermal conductivity.

The inner volume of this container is 1 × 10 (the ratio of the inner volume to the volume of the aluminum nitride molded body (internal volume / volume of the molded body)).
⁰ to 1 × 10 ⁷ is good. If a larger volume is used,
The carbon vapor pressure near the sample is low, and the effect of carbon to remove the grain boundary phase is reduced.

As for the sintering time, conventionally, sintering has been performed in a short time of 1 to 3 hours using various auxiliaries.
Even if it is fired in the above firing vessel, it is possible to densify the aluminum nitride sintered body and fix the oxygen on the surface of the raw material powder to the grain boundary phase, but the grain boundaries are formed between the AlN grains and at the triple point. There is a phase, and an AlN single phase sintered body cannot be obtained.
In addition, when a firing vessel that does not provide a carbon gas atmosphere as described above is used, the effect of removing the grain boundary phase does not appear even after firing for a long time. In order to make a substantially AlN single phase, it is most desirable that the time be 12 hours or more, depending on the sintering aid.

The firing temperature is preferably from 1550 to 2050 ° C. 1550
When firing at a temperature lower than ℃, a dense sintered body cannot be obtained depending on the particle size of the raw material powder and the amount of oxygen. Further, the generation of carbon gas from the firing vessel is reduced, and the grain boundary phase remains. Also, when firing at a temperature higher than 2050 ° C, the vapor pressure of AlN itself increases, making it difficult to densify, and a sub-phase, which is presumed to be a nitride of an additional element, remains in the sintered body, resulting in heat conduction. The rate may decrease.

Next, an example of the method for producing the aluminum nitride sintered body of the present invention will be described below.

First, a predetermined amount of an additive is added to AlN powder, and then mixed using a ball mill or the like. A normal pressure sintering method is used for sintering. In this case, a binder is added to the mixed powder, kneading, granulation, and sizing are performed, followed by molding. As a molding method, a die press, an isostatic press, a sheet molding or the like can be applied. Subsequently, the molded body is heated in a non-oxidizing atmosphere, for example, in a nitrogen gas stream to remove the binder, and then sintered at normal pressure. The firing vessel used at this time creates a carbon gas atmosphere during firing. For example, a carbon container having a ratio of the volume of the container to the volume of the molded body of 1 × 10 ¹ to 1 × 10 ⁷ is used. The sintering temperature is set to 1550-2050 ° C, and the sintering time is set to 4 hours or more. The sintered body of the present invention can be obtained by such a method.

Next, the effect of improving the thermal conductivity of the aluminum nitride sintered body of the present invention and the purifying action of the aluminum nitride sintered body by removing grain boundaries such as the (additive element) -Al-O-based compound phase will be described. The precise mechanism has not been completely elucidated at present, but according to the study of the present inventors, it is estimated as follows as a factor for increasing the thermal conductivity.

The first is the effect of trapping impurity oxygen in the AlN raw material powder by the additive. That is, by adding an alkaline earth metal and a rare earth element compound, impurity oxygen is fixed to the ridge and triple point of the AlN flow field in the form of an (additive element) -Al-O compound or the like. Oxygen solid solution is prevented, and the formation of Al oxynitride (AlON) and AlN polytype (27R type) is prevented. According to the research results of the inventors, it is known that the sintered bodies in which AlON and 27R are formed have low thermal conductivity. Suppressing the cause of such low thermal conductivity can be cited as one of the causes of high thermal conductivity.

For example, an impurity oxygen of the raw material powder if as an additive element chose Y _{is, 3Y 2 O 3 · 5Al 2} O 3, Y 2 O 3 · Al 2 O 3, 2Y 2 O 3 · Al 2
Trapped as compounds such as O ₃ and Y ₂ O ₃ . This state occurs at the initial stage of sintering, that is, in the sintering time of 0 to 3 hours, and the thermal conductivity reaches a maximum of about 190 w / mK.

In the subsequent sintering process, the carbon atmosphere reduces the grain boundary phase and starts to remove the grain boundary phase. Gradually, the grain boundary phase does not exist in the aluminum nitride sintered body and moves out of the sintered body. Finally, the sintered body becomes an AlN single phase substantially containing no other phases, and the thermal conductivity increases significantly. This is because the grain boundary phase having low thermal conductivity and acting as thermal resistance is removed. In addition, the particles of the sintered body grow by firing for a long time. When AlN particles grow, it means that the number of grain boundaries that become thermal resistance will eventually decrease,
A sintered body with small phonon scattering is obtained.

In addition to the above-mentioned removal of the subphase and grain growth,
By firing for a long time in a reducing atmosphere, it is also possible to purify AlN crystal grains, for example, to increase the thermal conductivity by reducing lattice defects.

(Example of the invention)

 Next, examples of the present invention will be described.

Example 1 0.86% by weight of oxygen was contained as an impurity, and the average particle size was 1.
72 .mu.m (centrifugal sedimentation diameter, Horiba CAPA-500 used, the dispersion medium n- butyl alcohol) average particle diameter Y ₂ O ₃ of 0.9 .mu.m 5 wt% (Y 39 wt% in terms of) the AlN powder additives Then, 1% by weight (0.40% by weight in terms of Ca) of CaCO ₃ having an average particle size of 2.0 μm was added, and the mixture was mixed using a ball mill to prepare a raw material. Then, 4 wt% of an organic binder was added to the raw material, and the mixture was granulated, followed by press molding at a pressure of 500 kg / cm ² to obtain a compact of 38 × 38 × 10 mm. This green compact was heated to 700 ° C. in a nitrogen gas atmosphere to remove the binder. Further, the degreased body was accommodated in a carbon container (firing container A) ground with an AlN plate coated with BN powder as a bottom plate. At this time, the shape and size of the container A is 12 cm
It is φ × 6.4cm and the inner volume is about 720cm ² . That is, the ratio of the volume of the container A to the volume of the AlN compact is about 5 × 10 ¹ . Using this container, sintering was carried out at 1900 ° C. for 96 hours in a nitrogen gas atmosphere (1 atm). AlN obtained
The density of the sintered body was measured. Also, from the sintered body, 10mm in diameter,
A 3.3 mm thick disk was ground and used as a test piece to measure the thermal conductivity by the laser flash method (manufactured by Vacuum Riko).
TC-3000 used). The measurement temperature is 25 ° C.

Table 1 shows the characteristics of the sintered body obtained under the above sintering conditions. Also, X-ray diffraction of this sintered body (Rotorflex RU-200 manufactured by Rigaku Denki, goniometer CN2173D5, radiation source Cu50k
V, 100 mA) is shown in FIG. 1, and an SEM photograph of the fracture surface of the sintered body is shown in FIG. 2 (using JSM-T20 manufactured by JEOL Ltd.).

Examples 2 to 27 By adding various additives to the AlN powder used in Example 1,
In addition, various sintered bodies were obtained in the same manner as in Example 1 by changing the firing time and temperature. The evaluation results are shown in Table 1.

Examples 28 to 37 Example 1 by changing the composition and pressure of the firing atmosphere
Various sintered bodies were obtained in the same manner as described above. The evaluation results of the sintered body are shown in Table 1.

Examples 38 to 43 Various sinters were obtained in the same manner as in Example 1, using various AlN powders and changing the firing conditions, the composition of the additives, and the amounts thereof. The evaluation results of the sintered body are shown in Table 1.

Examples 44 to 50 Various sintered compacts were obtained by the same method as in Example 1 except that the volume ratio between the container and the molded article was different from the molded article obtained in the same manner as in Example-1. Table 2 shows the evaluation results of the sintered body.

Example 48 An Al container was prepared in the same manner as in Example 1 except that a carbon container (fired container B) ground with a BN plate as a bottom plate was used.
An N sintered body was manufactured. The same evaluation is performed. Table-Results
2 is shown.

Example 49 An AlN sintered body was manufactured in the same manner as in Example 1 except that a container made entirely of carbon (sintering container C) was used. The same evaluation was performed and the results are shown in Table 2.

Example 50 In the carbon container (43 × 44 × 15 mm) used in Example 46, a carbon powder having an average particle size of 0.02 μm was packed, and the same compact as in Example 1 was placed therein at 1900 ° C. It was baked for 96 hours. The obtained sintered body was evaluated in the same manner as in Example 1, and the results are shown in Table 2.

Comparative Examples 1-3 AlN degreased bodies obtained by the same method as in Example 1 except that the types and amounts of the AlN powder and the additives were different were used for sintering containers.
A, variously set to B and C, 1900 ° C., 2 hr, then normal pressure sintering in a N ₂ atmosphere to obtain a sintered body. Table 3 shows the properties of these sintered bodies. Further, using the sintered body of Comparative Example 1, X
Figure 3 shows the results of X-ray diffraction, and shows the SEM of the fracture surface of the sintered body.
The photograph is shown in FIG. From these results and the result of the same evaluation, a compound containing yttrium was observed as a subphase, and it was found that the compound was not an AlN single phase. As a result, the thermal conductivity was a low value of 170 w / mk or less.

When the sintering time is as short as less than 4 hours, it is understood that the removal of the grain boundary phase by using the carbon container is not sufficient, and it takes a long time to obtain an AlN sintered body having high thermal conductivity ( It can be seen that sintering for more than 4 hours is necessary.

Comparative Examples 4 to 6 An AlN degreased body obtained by the same method as in Example 1 was used. In Comparative Example 4, the whole inner container was made of AlN (sintering container D). In Comparative Example 5, the entire inner member was made of alumina. In the container (sintering container E), in Comparative Example 6, the inside was entirely made of tungsten (sintering container F), and was sintered under normal pressure at 1900 ° C. for 96 hours in a stream of N ₂ to obtain a sintered body. Was. Table 3 shows the properties of these sintered bodies. Further, the result of X-ray diffraction using the sintered body of Comparative Example 4 is shown in FIG. From these results and the results of the evaluation, a compound containing yttrium as a subphase was observed, and it was found that the compound was not an AlN single phase. As a result, the thermal conductivity is also a relatively low value of 168 w / mK or less.

As described above, even when a sintering vessel made of carbon at least in part is not used, an AlN sintered body having high thermal conductivity cannot be obtained, and the effectiveness of the carbon atmosphere can be understood.

Comparative Example 7 The AlN powder used in Example 1 was press-molded at a pressure of 500 kg / cm ³ to obtain a green compact of 30 × 30 × 10 mm. The green compact was placed in a carbon mold and placed in a nitrogen gas atmosphere. , Temperature 1900 ℃, 400
Hot press sintering was performed under a pressure of kg / cm ³ for 1 hour to obtain a sintered body. Table 3 shows the characteristics of the sintered body. Further results of X-ray diffraction are shown in FIG. From this result, an Al-ON-based compound was observed as a subphase, and it was found that the compound was not an AlN single phase. As a result, the thermal conductivity was as low as 80 w / mK.

If the rare earth and alkaline earth metal element compounds are not added in this way, the impurity oxygen on the surface of the AlN raw material powder reacts with the AlN to form an Al-ON compound that impedes heat conduction. We understand effectiveness.

[Effects of the Invention] As described above, the aluminum nitride sintered body of the present invention is substantially composed of an AlN single phase and has excellent properties such as high purity and high thermal conductivity. ,
Its industrial value is extremely large.

[Brief description of the drawings]

FIGS. 1, 3, 5 and 6 are X-ray diffraction pattern diagrams of the sintered body, and FIGS. 2 and 4 show the crystal structure of the fracture surface of the sintered body (by SEM photograph). FIG. 1 ... diffraction peak of AlN 2 ... diffraction peak of Y-Al-O compound 3 ... peak of Al-ON compound 4 ... AlN particles 5 ... Y-Al-O compound (grain boundary phase)

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshiko Sato 1 Toshiba-cho, Komukai-shi, Kawasaki-shi Toshiba Research Institute, Inc. (72) Inventor Akihiro Horiguchi 1 Toshiba-cho, Komukai-shi, Kawasaki-shi Toshiba Research Institute In-house (72) Inventor Akihiko Tsuge 1 Komukai Toshiba-cho, Yuki-ku, Kawasaki-shi Toshiba Research Institute, Inc. (56) References JP-A-62-132776 (JP, A)

Claims (3)

(57) [Claims] 1. A molded article comprising (a) an aluminum nitride as a main component and an additive comprising an alkaline earth metal compound and a rare earth compound added thereto in an amount of 0.05 to 25% in terms of the weight of each element, or The molded body is fired at 1550 to 2050 ° C. for less than 4 hours, and a sintered body containing a constituent phase other than AlN is fired. (B) A firing vessel for generating a carbon gas and / or a firing vessel for generating a carbon gas during firing (C) the volume ratio of the internal volume of the firing vessel to the compact or sintered compact is 1 × 10 ⁰ to 1 × 10 ^{7 and} at 1550-2050 ° C
A method for producing a highly thermally conductive aluminum nitride sintered body, characterized in that the sintered body is fired for 12 hours or more to obtain a sintered body having a thermal conductivity exceeding 230 W / mk which is substantially free of constituent phases other than AlN. 2. The method according to claim 1, wherein the alkaline earth metal element is at least one of Ca, Sr, and Ba, and the rare earth element is at least one of Y, Sc, Dy, and Ce. Of manufacturing a highly thermally conductive aluminum nitride sintered body of the present invention. 3. The high thermal conductivity according to claim 1, wherein the density of the sintered body is 3.120 to 3.285 g / cm ³ , and the thermal conductivity at 25 ° C. is 230 W / m · k or more. Of producing a sintered aluminum nitride sintered body.